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  • 1.
    Ann Delgado, Alexis Antoinette
    et al.
    Southern Methodist Univ, Dept Chem, Dallas, TX 75275 USA..
    Sethio, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Organic Chemistry.
    Matthews, Devin
    Southern Methodist Univ, Dept Chem, Dallas, TX 75275 USA..
    Oliveira, Vytor
    Inst Tecnol Aeronaut ITA, Dept Quim, Sao Jose Dos Campos, SP, Brazil..
    Kraka, Elfi
    Southern Methodist Univ, Dept Chem, Dallas, TX 75275 USA..
    Substituted hydrocarbon: a CCSD(T) and local vibrational mode investigation2021In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 119, no 21-22, article id e1970844Article in journal (Refereed)
    Abstract [en]

    Substituent effects on the carbon-carbon bonds of hydrocarbons have been a topic of interest within the past seven decades as resultant information would enable one to tune the activity of CC bonds. However, current assessments of the C equivalent to C, C=C, and C-C bond strength of acetylene, ethylene, and ethane as well as their derivatives rely on indirect measures such as bond length and bond dissociation enthalpy. In this work, we introduce a quantitative measure of the intrinsic strength of C equivalent to C, C=C, and C-C bonds for a set of 40 hydrocarbon systems consisting of 3 parent structures, 36 hydrocarbon derivatives involving CF3, CH3, CHO, F, NH2, or OH groups, and a conjugated system, based on vibrational spectroscopy. Local mode force constants k(a)(CC) were computed at the CCSD(T)/cc-pVTZ level of theory for systems 1-32 and 34-40 and CCSD(T)/cc-pVDZ for 33. From k(a)(CC), we derived related bond strength orders BSO n(CC) in order to provide quantitative measures of intrinsic bond strength. Topological electron density and natural population analyses were carried out as to analyze the nature of these bonds and complement bond strength measures. For substituted hydrocarbon systems, we found the strengthening/weakening of the CC bonds occurs as the covalent nature of the bond increases/decreases by means of varying charge delocalizations. Our findings provide new guidelines for desirably modulating C equivalent to C, C=C, and C-C bond strength and for the design of prospective pathways for bond cleavage reactions. [GRAPHICS] .

  • 2. Aquilante, Francesco
    et al.
    Cossi, M
    Crescenzi, O
    Scalmani, G
    Barone, V
    Computation of the acetone ultraviolet spectrum in gas phase and in aqueous solution by a mixed discrete/continuum model.2003In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 101, no 13, p. 1945-1953Article in journal (Refereed)
    Abstract [en]

    The ultraviolet spectrum of acetone in vacuum and in aqueous solution has been computed by different quantum mechanical methods coupled to the polarizable continuum model (PCM) for describing bulk solvent effects. The results in vacuo show that the time-dependent density functional theory (TDDFT) approach using the PBE0 functional reproduces quite well the result obtained at the CASPT2 level. Supermolecule computations confirm that water molecules belonging to the first shell of polar groups ( here the carbonyl oxygen) must be explicitly included in the quantum mechanical treatment, whereas the effect of other solvent molecules ( which is far from being negligible) can be reliably described by the PCM. Moreover, statistical averaging effects have been taken into account by performing canonical molecular dynamics (MD) simulations followed by TDDFT quantum mechanical computations on representative clusters of increasing dimensions immersed in a polarizable continuum. The results show that the combined MD/DFT/PCM approach is reliable and effective, although the performances of the force field used in the MD simulations must be further investigated.

  • 3.
    Aquilante, Francesco
    et al.
    Univ Bologna, Dipartimento Chim G Ciamician, Bologna, Italy..
    Delcey, Mickael G.
    Lawrence Berkeley Natl Lab, Div Chem Sci, Berkeley, CA USA.;Univ Calif Berkeley, Dept Chem, Kenneth S Pitzer Ctr Theoret Chem, Berkeley, CA 94720 USA..
    Pedersen, Thomas Bondo
    Univ Oslo, Dept Chem, Ctr Theoret & Computat Chem, Oslo, Norway..
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry. Uppsala Univ, Dept Chem Angstrom, Theoret Chem Programme, Uppsala, Sweden..
    Inner projection techniques for the low-cost handling of two-electron integrals in quantum chemistry2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 17-18, p. 2052-2064Article in journal (Refereed)
    Abstract [en]

    The density-fitting technique for approximating electron-repulsion integrals relies on the quality of auxiliary basis sets. These are commonly obtained through data fitting, an approach that presents some shortcomings. On the other hand, it is possible to derive auxiliary basis sets by removing elements from the product space of both contracted and primitive orbitals by means of a particular form of inner projection technique that has come to be known as Cholesky decomposition (CD). This procedure allows for on-the-fly construction of auxiliary basis sets that may be used in conjunction with any quantum chemical method, i.e. unbiased auxiliary basis sets. One key feature of these sets is that they represent the electron-repulsion integral matrix in atomic orbital basis with an accuracy that can be systematically improved. Another key feature is represented by the fact that locality of fitting coefficients is obtained even with the long-ranged Coulomb metric, as result of integral accuracy. Here we report on recent advances in the development of the CD-based density fitting technology. In particular, the implementation of analytical gradients algorithms is reviewed and the present status of local formulations - potentially linear scaling - is analysed in detail.

  • 4.
    Bao, Jie J.
    et al.
    Univ Minnesota, Chem Theory Ctr, Dept Chem, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Hermes, Matthew R.
    Univ Chicago, James Franck Inst, Pritzker Sch Mol Engn, Dept Chem,Chicago Ctr Theoret Chem, 5640 S Ellis Ave, Chicago, IL 60637 USA.;Argonne Natl Lab, Lemont, IL 60439 USA..
    Scott, Thais R.
    Univ Chicago, James Franck Inst, Pritzker Sch Mol Engn, Dept Chem,Chicago Ctr Theoret Chem, 5640 S Ellis Ave, Chicago, IL 60637 USA.;Argonne Natl Lab, Lemont, IL 60439 USA..
    Sand, Andrew M.
    Butler Univ, Dept Chem & Biochem, Indianapolis, IN 46208 USA..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Organic Chemistry.
    Gagliardi, Laura
    Univ Chicago, James Franck Inst, Pritzker Sch Mol Engn, Dept Chem,Chicago Ctr Theoret Chem, 5640 S Ellis Ave, Chicago, IL 60637 USA.;Argonne Natl Lab, Lemont, IL 60439 USA..
    Truhlar, Donald G.
    Univ Minnesota, Chem Theory Ctr, Dept Chem, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Analytic gradients for compressed multistate pair-density functional theory2022In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 120, no 19-20, article id e2110534Article in journal (Refereed)
    Abstract [en]

    Photochemical reactions often involve states that are closely coupled due to near degeneracies, for example by proximity to conical intersections. Therefore, a multistate method is used to accurately describe these states; for example, ordinary perturbation theory is replaced by quasidegenerate perturbation theory. Multiconfiguration pair-density functional theory (MC-PDFT) provides an efficient way to approximate the full dynamical correlation energy of strongly correlated systems, and we recently proposed compressed multistate pair-density functional theory (CMS-PDFT) to treat closely coupled states. In the present paper, we report the implementation of analytic gradients for CMS-PDFT in both OpenMolcas and PySCF, and we illustrate the use of these gradients by applying the method to the excited states of formaldehyde and phenol.

  • 5. Bernhardsson, Anders
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Olsen, Jeppe
    Fülscher, Markus
    A direct implementation of the second-order derivatives of multiconfigurational SCF energies and an analysis of the preconditioning in the associated response equation1999In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 96, no 4, p. 617-628Article in journal (Refereed)
    Abstract [en]

    A direct implementation of the reduced multiplication scheme of the Rys-Gauss quadrature in the computation of the second-order geometric derivatives for MCSCF wavefunctions is presented. Characteristics unique to this implementation are simultaneous evaluation of first- and second-order two-electron integral derivatives, compact representation of intermediate elements and efficient prescreening. Finally, a novel technique for the preconditioning of the response equations is reported. Applications sizing up to 510 basis functions are included in the presentation.

  • 6.
    Brändas, Erkki
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Per-Olov Löwdin - father of quantum chemistry2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 17-18, p. 1995-2024Article in journal (Other academic)
    Abstract [en]

    During 2016, we celebrate the 100th anniversary of the birth of Per-Olov Lowdin. He was appointed to the first Lehrstuhl in quantum chemistry at Uppsala University in 1960. Lowdin introduced quantum chemistry as a field in its own right by formulating its goals, establishing fundamental concepts, like the correlation energy, the method of configuration interaction, reduced density matrices, natural spin orbitals, charge and bond order matrices, symmetric orthogonalisation, and generalised self-consistent fields. His exposition of partitioning technique and perturbation theory, wave and reaction operators and associated non-linear summation techniques, introduced mathematical rigour and deductive order in the interpretative organisation of the new field. He brought the first computer to Uppsala University and pioneered the initiation of electronic brains' and anticipated their significance for quantum chemistry. Perhaps his single most influential contribution to the field was his education of two generations of future faculty in quantum chemistry through Summer Schools in the Scandinavian Mountains, Winter Institutes at Sanibel Island in the Gulf of Mexico. Per-Olov Lowdin founded the book series Advances in Quantum Chemistry and the International Journal of Quantum Chemistry. The evolution of quantum chemistry is appraised, starting from a collection of cross-disciplinary applications of quantum mechanics to the technologically advanced and predominant field of today, virtually used in all branches of chemistry. The scientific work of Per-Olov Lowdin has been crucial for the development of this new important province of science. [GRAPHICS] .

  • 7.
    Brändas, Erkki
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Dunne, Lawrence J.
    Bardeen-Cooper- Schrieffer (BCS) theory and Yang's concept of off- diagonal long- range order (ODLRO)2014In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 112, no 5-6, p. 694-699Article in journal (Refereed)
    Abstract [en]

    In this contribution, we pay tribute to the scientific achievements of Professor Rodney J. Bartlett on account of his seminal contributions to the many-body electron correlation problem. We are here concerned with strongly correlated situations as met in the theory of superconductivity. In condensed matter physics, one often makes use of the famous Bardeen-Cooper-Schrieffer (BCS) formulation, while quantum chemists often instigate an approach that originates in Yang's concept of off-diagonal long-range order (ODLRO), and the Coleman-Sasaki extreme state. Our aim is to demonstrate that both approaches are essentially equivalent by deriving the BCS gap equation from the assumption of the presence of ODLRO.

  • 8.
    Brändas, Erkki J.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Molecular theory of the genetic code2018In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 116, no 19-20, p. 2622-2632Article in journal (Refereed)
    Abstract [en]

    This article honours Michael Baer on the occasion of his 80th birthday and celebrates his scientific contributions to non-adiabatic chemical physics. This undertaking prompts the presentation of a first principles molecular theory of the genetic code. Jacques Monod's classic essay, 'Chance and Necessity', is exercised as a platform for this discussion. In particular the controversial concept of teleonomy is considered and evaluated in relation to modern developments in chemical physics.

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  • 9.
    Brändas, Erkki J.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry.
    Partitioning technique for open systems2010In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 108, no 21-23, p. 3259-3272Article in journal (Refereed)
    Abstract [en]

    The focus of the present contribution is essentially confined to three research areas carried out during the author's turns as visiting (assistant, associate and full) professor at the University of Florida's Quantum Theory Project, QTP. The first two topics relate to perturbation theory and spectral theory for self-adjoint operators in Hilbert space. The third subject concerns analytic extensions to non-self-adjoint problems, where particular consequences of the occurrence of continuous energy spectra are measured. In these studies general partitioning methods serve as general cover for perturbation-, variational- and general matrix theory. In addition we follow up associated inferences for the time dependent problem as well as recent results and conclusions of a rather general yet surprising character. Although the author spent most of his times at QTP during visits in the 1970s and 1980s, collaborations with department members and shorter stays continued through later decades. Nevertheless the impact must be somewhat fragmentary, yet it is hoped that the present account is sufficiently self-contained to be realistic and constructive.

  • 10.
    Fernández Galván, Ignacio
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Xiao, Hong-Yan
    Navizet, Isabelle
    Liu, Ya-Jun
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    S0 → S3 transition in recombination products of photodissociated dihalomethanes2014In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 112, no 5-6, p. 575-582Article in journal (Refereed)
    Abstract [en]

    Species of the form CH2X–Y (X, Y = Br, I) have been proposed and identified as recombination products of the photodissociation of the parent dihalomethanes. Second-order complete active space perturbation theory (CASPT2) calculations of the vertical absorption energies considerably overestimate the experimental transient absorption band maxima, while the computationally cheaper time-dependent density functional theory (TD-DFT) method yields results with a reasonable agreement. In this work, we try to find the reason for this unexpected performance difference. In an initial study of the I2 molecule, we establish that CASPT2 is capable of providing quantitatively accurate results and that the TD-DFT values are only valid at first sight, but are qualitatively flawed. In the CASPT2 calculations for the CH2X–Y molecules, we include relativistic corrections, spin–orbit coupling, vibrational and thermal effects, and the solvent polarisation. Unfortunately, the results do not improve appreciably compared to the experimental measurements. We conclude that the good agreement of TD-DFT results is very likely fortuitous in this case as well, and that further theoretical and experimental investigations are probably needed to resolve the current discrepancy between CASPT2 and experiments.

  • 11. Gusarov, Sergey
    et al.
    Malmqvist, Per-Åke
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Using on-top pair density for construction of correlation functionals for multideterminant wave functions2004In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 102, no 21-22, p. 2207-2216Article in journal (Refereed)
    Abstract [en]

    The value of the two-particle density function at coalescence P-2(ontop)(r) = rho(2)(r, r) is frequently used as an additional variable for formulating approximate exchange-correlation or correlation functionals. Here, its applications as one of the key variables for the construction of new DFT (preferably multi-determinant) functionals is investigated. The basic formalism is presented and it is shown that this replacement avoids some difficulty to construct a Fock matrix in a ROKS (restricted open-shell Kohn-Sham) method and also to reduce the ‘double counting’ of correlation energy in CASDFT (complete active space density functional theory) calculations. Calculations of excitation energies for transition metals and dissociation curves for diatomic molecules are presented as an example.

  • 12.
    Head-Gordon, Martin
    et al.
    Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    A celebration of the Swedish school2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 17-18, p. 1993-1994Article in journal (Other academic)
  • 13.
    Hermansson, Kersti
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry.
    Lindgren, Jan
    Ågren, Hans
    THEORETICAL-STUDIES OF IR INTENSITIES IN ZN2+OH2 AND MG2+OH21986In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 57, no 4, p. 857-863Article in journal (Refereed)
  • 14. Polyak, Iakov
    et al.
    Jenkins, Andrew J.
    Vacher, Morgane
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Bouduban, Marine E. F.
    Bearpark, Michael J.
    Robb, Michael A.
    Charge migration engineered by localisation: electron-nuclear dynamics in polyenes and glycine2018In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 116, no 19-20, p. 2474-2489Article in journal (Refereed)
    Abstract [en]

    We demonstrate that charge migration can be ‘engineered’ in arbitrary molecular systems if a single localised orbital – that diabatically follows nuclear displacements – is ionised. Specifically, we describe the use of natural bonding orbitals in Complete Active Space Configuration Interaction (CASCI) calculations to form cationic states with localised charge, providing consistently well-defined initial conditions across a zero point energy vibrational ensemble of molecular geometries. In Ehrenfest dynamics simulations following localised ionisation of -electrons in model polyenes (hexatriene and decapentaene) and -electrons in glycine, oscillatory charge migration can be observed for several femtoseconds before dephasing. Including nuclear motion leads to slower dephasing compared to fixed-geometry electron-only dynamics results. For future work, we discuss the possibility of designing laser pulses that would lead to charge migration that is experimentally observable, based on the proposed diabatic orbital approach.

    KEYWORDS: Ehrenfest method, coupled electron-nuclear dynamics, charge migration, localised orbital

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  • 15. Schütz, Martin
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Werner, Hans-Joachim
    Integral-direct electron correlation methods1999In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 96, no 4, p. 719-733Article in journal (Refereed)
    Abstract [en]

    Integral-direct implementations of Moller-Plesset perturbation theory up to fourth order (MP4(SDQ) without triple excitations), quadratic configuration interaction with single and double excitations (QCISD), coupled cluster (CCSD), multiconfiguration self-consistent field (MCSCF), second- and third-order multireference perturbation theory (MRPT2, MRPT3), and internally contracted multireference configuration interaction (MRCI) methods are described. All methods use the same three integral-direct kernels, namely direct calculation of Fock matrices, direct partial integral transformation, and direct evaluation of external exchange matrices. New algorithms for the latter two procedures are proposed and implemented, for which the memory requirement scales only linearly with basis set size. Test calculations with more than 500 basis functions are presented. It is demonstrated that integral prescreening can significantly reduce the scaling of the computational cost with molecular size N. For small polyglycine peptides (Gly)(n) (n = 1-4), we observe a scaling of N-3 for the direct integral transformation (formal scaling N-5), and N-4 for the direct computation of the external exchange matrices (formal scaling N-6).

  • 16.
    Siretskiy, Alexey
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
    Elvingson, Christer
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
    Role of non-uniform confinement in shape transitions of semi-stiff polymers2013In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 111, no 1, p. 101-109Article in journal (Refereed)
    Abstract [en]

    New types of nanostructures are constantly being developed synthetically but are also found in biological systems. Specific examples include the production of carbon nanocones as well as the conical core in some viruses. Such conical structures can be used to investigate the role of non-uniform confinement on the stability of e.g. toroidal structures formed by semi-stiff circular polymers, such as DNA. In this communication we are interested in the principal features of the compaction process. Using an external field and a conical confinement we observe several distinct shape transitions from a circle-like shape to several toroidal-like loops for both a two-dimensional and a three-dimensional system. The thermodynamic stability of these toroidal-like structures was investigated by evaluating their relative free energies using Monte Carlo simulations in the Extended Ensemble in the case of a two-dimensional system and by observing a hysteresis of the compaction-extension curve for the three-dimensional case.

  • 17. Stålring, Jonna
    et al.
    Bernhardsson, Anders
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Analytical gradients of a state average MCSCF state and a state average diagnostic2001In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 99, no 2, p. 103-114Article in journal (Refereed)
    Abstract [en]

    An efficient method for calculating the Lagrange multipliers and the analytical gradients of one state included in a state average MCSCF wave function is presented. It is demonstrated that the state average energy of an ‘equal-weight’ scheme is invariant to rotations within the state average subspace and that the corresponding rotations should be eliminated from the Lagrangian equations. Finally, a diagnostic is presented, which gauges the energy difference between a state defined by a state average calculation and the corresponding fully variational multi-configurational SCF state.

  • 18. Stålring, Jonna
    et al.
    Gagliardi, Laura
    Malmqvist, Per-Åke
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    A theoretical study of the 2(1)A(g)<- 1(1)A(g) two-photon transition and its vibronic band in trans-stilbene2002In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 100, no 11, p. 1791-1796Article in journal (Refereed)
    Abstract [en]

    The two-photon spectrum of the 2(1)A(g)–>1(1)A(g) transition in trans-stilbene has been calculated at the complete active space self-consistent field (CASSCF) level of theory. Energies were obtained at the complete active space second-order perturbation (CASPT2) level of theory, while the geometries of both the initial and final states were optimized at the CASSCF level. The energy and the geometry optimizations were performed using an active space of 14 electrons in 14 active pi orbitals. The vibrational frequencies of both states and the two-photon transition (TPT) cross-section were calculated with a smaller active space where the two lowest pi orbitals were kept inactive. A newly implemented algorithm, in the quantum chemical package Molcas was used to determine the two-photon transition intensity. This method requires only the linear response of the CASSCF wavefunction. Furthermore, the vibronic structure of this TPT was studied. The Franck-Condon factors were obtained by calculating the overlap between the vibrational states involved, which were determined from the force fields of both the initial and final states, at the CASSCF level of theory. The results are in agreement with experiment.

  • 19. Stålring, Jonna
    et al.
    Schütz, Martin
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Karlström, Gunnar
    Widmark, Per-Olof
    The ammonia dimer equilibrium dissociation energy: convergence to the basis set limit at the correlated level2002In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 100, no 21, p. 3389-3399Article in journal (Refereed)
    Abstract [en]

    The low-energy region of the intermolecular potential energy hypersurface (PES) of the ammonia dimer was studied at the level of second-order Moller-Plesset perturbation theory (MP2) using a very large basis set. Individual minima were located on the PES employing the counterpoise (CP) correction to account for the basis set superposition error (BSSE). Apart from these canonical MP2 calculations local MP2 (LMP2) calculations were performed. For the latter the BSSE at the correlated level is inherently absent by virtue of the local truncation of the virtual space. Results from canonical and local MP2 calculations are compared and the reliability of the LMP2 method for intermolecular complexes and clusters is discussed. The canonical MP2 calculations predicted five minimum structures, the four most stable ones lying energetically very close. For these four structures single point MP2 energy calculations with a further extended basis set (1024 functions for the ammonia dimer) were performed. The equilibrium dissociation energies so obtained are close to the one-particle basis set limit, as illustrated by a remaining BSSE of less than 0.2 kJ mol(-1). The geometry optimizations at the LMP2 level, using the three most stable canonical MP2 structures as initial geometries, all collapsed to a single minimum corresponding to an asymmetric structural arrangement. A canonical MP2 single point calculation, at that geometry, revealed that the LMP2 minimum structure is virtually as stable as the lowest minima on the canonical MP2 PES. Based on these calculations the global minimum of the ammonia dimer was assigned to a part of the PES represented by an asymmetric structure with an equilibrium dissociation energy of 13.5 +/- 0.3 kJ mol(-1.)

  • 20.
    Sørensen, Lasse Kragh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Guo, Meiyuan
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Applications to metal K pre-edges of transitionmetal dimers illustrate the approximate origin independence for the intensities in the length representation2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 1-2, p. 174-189Article in journal (Refereed)
    Abstract [en]

    X-ray absorption spectroscopy (XAS) in the metal K pre-edge is a standard probe of electronic and geometric structure of transition metal complexes. Simulating the K pre-edge spectra requires contributions beyond the electric dipole, but if that term is non-zero, the second-order terms, e. g. electric quadrupoles, are no longer origin-independent. In the velocity representation, complete origin independence can be achieved by including all terms to the same order in the oscillator strength. Here, we implement that approach in the length representation and use it for restricted active space (RAS) simulations of metal K pre-edges of iron monomers and dimers. Complete origin independence is not achieved and the size of the remaining errors depends on the electric dipole oscillator strength and its ratio in length and velocity representations. The error in the origin independence is in the ANO basis sets two orders of magnitude smaller than the value of the individual contributions. For systemswith strong electric dipole contributions, the errors are not significant within 3 angstrom from a metal centre, far enough to handlemany multi-metal systems. Furthermore, we discuss the convergence of the multipole expansion, the possibility to assign spectral contributions, and the origin of negative absorption intensities. [GRAPHICS]

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  • 21.
    Sørensen, Lasse Kragh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry. Aarhus Univ, qLEAP Ctr Theoret Chem, Dept Chem, Aarhus, Denmark..
    Olsen, Jeppe
    Aarhus Univ, qLEAP Ctr Theoret Chem, Dept Chem, Aarhus, Denmark..
    Collecting all intermediates with an optimal scaling for the generalised-active-space coupled-cluster method with application to SbH2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 1-2, p. 90-108Article in journal (Refereed)
    Abstract [en]

    A string-based coupled-cluster method of general excitation rank, with optimal scaling and easy collection of all possible intermediates, which accounts for special relativity within the four-component framework is presented. The easy identification and collection of intermediates is achieved by extending the excitation-class formalism and introducing a contraction order for the operator indices. Initially, all possible contractions between the Hamiltonian and the intermediateswith the cluster operator are found, stored and used for driving the algorithm, thereby defining an algorithm which allows for very complex state-selective, multi-reference, coupled-cluster calculations. The algorithm for solving the coupled-cluster equations can be used in both relativistic as well as non-relativistic calculations by appropriate restrictions in the excitation-class formalism. The capabilities of the new method are demonstrated in the application to the electronic ground state of the SbH molecule. In these calculations simulated multi-reference expansions with double, triple and quadruple excitations into the external space as well as the regular coupled-cluster hierarchy up to full quadruples excitations are compared. [GRAPHICS]

  • 22.
    Valiev, R. R.
    et al.
    KTH Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem & Biol, Stockholm, Sweden.;Tomsk State Univ, Dept Optic & Spect, Tomsk, Russia.;Tomsk Polytechn Univ, Dept Gen Chem & Chem Engn, Tomsk 634050, Russia..
    Eland, John H. D.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics. Univ Gothenburg, Dept Phys, Gothenburg, Sweden.;Univ Oxford, Phys & Theoret Chem Lab, Chem Lab, Oxford, England..
    Feifel, Raimund
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics. Univ Gothenburg, Dept Phys, Gothenburg, Sweden..
    Ågren, H.
    KTH Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem & Biol, Stockholm, Sweden..
    Optimization of core-valence states of molecules2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 1-2, p. 252-259Article in journal (Refereed)
    Abstract [en]

    Core-valence double-electron ionisation spectra of a few small molecules - arbon monoxide, ammonia, methyl fluoride and thiophene - are presented and analysed using the self-consistent field algorithm introduced by Hans Jorgen Jensen 30 years ago. It confirms the utility of this algorithm, frequently employed to obtain stable and sharp convergence of wave functions for the benefit of calculations of a great variety of molecular properties, thus also for optimisation of core-valence states and for the interpretation of the corresponding spectra. [GRAPHICS]

  • 23.
    Yanai, Takeshi
    et al.
    Inst Mol Sci, Dept Theoret & Computat Mol Sci, Okazaki, Aichi, Japan.;Grad Univ Adv Studies, Sch Phys Sci, Dept Funct Mol Sci, Okazaki, Aichi, Japan..
    Kurashige, Yuki
    Kobe Univ, Grad Sch Syst Informat, Kobe, Hyogo, Japan.;PRESTO, Japan Sci & Technol Agcy, Kawaguchi, Saitama, Japan.;Kyoto Univ, Grad Sch Sci, Dept Chem, Kyoto, Japan..
    Saitow, Masaaki
    Grad Univ Adv Studies, Sch Phys Sci, Dept Funct Mol Sci, Okazaki, Aichi, Japan.;Max Planck Inst Chem Energiekonvers, Stiftstr 34-36, D-45470 Mulheim, Germany..
    Chalupsky, Jakub
    Acad Sci Czech Republ, Inst Organ Chem & Biochem, Prague, Czech Republic..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Malmqvist, Per-Åke
    Lund Univ, Dept Theoret Chem, Lund, Sweden..
    Influence of the choice of projection manifolds in the CASPT2 implementation2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 17-18, p. 2077-2085Article in journal (Refereed)
    Abstract [en]

    The Complete Active Space Second-Order Perturbation Theory (CASPT2) is well-established as a high-accuracy electronic structure method. It was originally implemented in the early 1990s to an efficient computer code in the molcas program suite, and this implementation has been extensively used as a standard tool. Here, we report a comparison of it against two independent computer-aided implementations of the CASPT2 method, revealing that the CASPT2 energies provided by the original code of molcas (version 8 or earlier) are inconsistent with the predictions of the lately developed computer-aided implementations. It is shown that this error is associated with the projections of the first-order equation onto the fully internally contracted multireference bases which are partially inconsistent between the left- and right-hand sides. The degree of the errors is assessed by performing illustrative CASPT2 calculations. The errors in total CASPT2 energies are demonstrated to be negligible relative to chemical accuracy in many cases, while there is a difficult case where they may substantially alter chemical description. The incorporation of the consistent projections into molcas has been carried out, which is available in the version 8 sp1.

1 - 23 of 23
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