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Extending the Reach of Accurate Wavefunction Methods
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Multiconfigurational quantum chemistry methods, and especially the multiconfigurational self-consistent field (MCSCF) and multireference perturbation theory (MRPT2), are powerful tools, particularly suited to the accurate modeling of photochemical processes and transition metal catalysis. However, they are limited by their high computational cost compared to other methods, especially density functional theory. Moreover, there are areas where they would be expected to perform well, but where they are not applied due to lack of experience.

This thesis addresses those issues. First, the efficiency of the Cholesky decomposition approximation to reduce the cost of MCSCF and MRPT2 without sacrificing their accuracy is demonstrated. This then motivates the extension of the Cholesky approximation to the computation of MCSCF nuclear gradients, thus strongly improving the ability to perform MCSCF non-adiabatic molecular dynamics. Typically, a tenfold speed-up is observed allowing dynamic simulation of larger systems or over longer times.

Finally, multiconfigurational methods are applied to the computation of X-ray spectra of transition metal complexes. The importance of the different parameters in the calculation is systematically investigated, laying the base for wider applications of those accurate methods in the modeling of X-ray spectroscopy. A tool to analyze the resulting spectrum in terms of molecular orbitals is also presented, strengthening the interplay between theory and experiments.

With these developments and other significant ones that have happened in recent years, multiconfigurational methods can now reach new grounds and contribute to important new discoveries

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1228
Keyword [en]
Quantum chemistry, Density fitting, CASSCF, Analytical gradients, Photochemistry, X-ray spectroscopy
National Category
Theoretical Chemistry
Research subject
Chemistry with specialization in Quantum Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-243573ISBN: 978-91-554-9168-0 (print)OAI: oai:DiVA.org:uu-243573DiVA: diva2:787594
Public defence
2015-03-31, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2015-03-10 Created: 2015-02-10 Last updated: 2015-04-14Bibliographically approved
List of papers
1. Calibration of Cholesky Auxiliary Basis Sets for Multiconfigurational Perturbation Theory Calculations of Excitation Energies
Open this publication in new window or tab >>Calibration of Cholesky Auxiliary Basis Sets for Multiconfigurational Perturbation Theory Calculations of Excitation Energies
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2010 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 6, no 3, 747-754 p.Article in journal (Refereed) Published
Abstract [en]

The accuracy of auxiliary basis sets derived from Cholesky decomposition of two-electron integrals is assessed for excitation energies calculated at the state-average complete active space self-consistent field (CASSCF) and multiconfigurational second order perturbation theory (CASPT2) levels of theory using segmented as well as generally contracted atomic orbital basis sets. Based on 196 valence excitations in 26 organic molecules and 72 Rydberg excitations in 3 organic molecules, the results show that Cholesky auxiliary basis sets can be used without compromising the accuracy of the multiconfigurational methods. Specifically, with a decomposition threshold of 10(-4) au, the mean error due to the Cholesky auxiliary basis set is 0.001 eV, or smaller, decreasing with increasing atomic orbital basis set quality.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-141205 (URN)10.1021/ct900612k (DOI)
Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2017-12-11Bibliographically approved
2. Analytical gradients of complete active space self-consistent field energies using Cholesky decomposition: Geometry optimization and spin-state energetics of a ruthenium nitrosyl complex
Open this publication in new window or tab >>Analytical gradients of complete active space self-consistent field energies using Cholesky decomposition: Geometry optimization and spin-state energetics of a ruthenium nitrosyl complex
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2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 17, 174103- p.Article in journal (Refereed) Published
Abstract [en]

We present a formulation of analytical energy gradients at the complete active space self-consistent field (CASSCF) level of theory employing density fitting (DF) techniques to enable efficient geometry optimizations of large systems. As an example, the ground and lowest triplet state geometries of a ruthenium nitrosyl complex are computed at the DF-CASSCF level of theory and compared with structures obtained from density functional theory (DFT) using the B3LYP, BP86, and M06L functionals. The average deviation of all bond lengths compared to the crystal structure is 0.042 angstrom at the DF-CASSCF level of theory, which is slightly larger but still comparable with the deviations obtained by the tested DFT functionals, e. g., 0.032 angstrom with M06L. Specifically, the root-mean-square deviation between the DF-CASSCF and best DFT coordinates, delivered by BP86, is only 0.08 angstrom for S-0 and 0.11 angstrom for T-1, indicating that the geometries are very similar. While keeping the mean energy gradient errors below 0.25%, the DF technique results in a 13-fold speedup compared to the conventional CASSCF geometry optimization algorithm. Additionally, we assess the singlet-triplet energy vertical and adiabatic differences with multiconfigurational second-order perturbation theory (CASPT2) using the DF-CASSCF and DFT optimized geometries. It is found that the vertical CASPT2 energies are relatively similar regardless of the geometry employed whereas the adiabatic singlet-triplet gaps are more sensitive to the chosen triplet geometry. (C) 2014 AIP Publishing LLC.

National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-227732 (URN)10.1063/1.4873349 (DOI)000336048000005 ()
Available from: 2014-06-30 Created: 2014-06-30 Last updated: 2017-12-05Bibliographically approved
3. Analytical gradients of the state-average complete active space self-consistent field method with density fitting
Open this publication in new window or tab >>Analytical gradients of the state-average complete active space self-consistent field method with density fitting
2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 4, 044110Article in journal (Refereed) Published
Abstract [en]

An efficient implementation of the state-averaged complete active space self-consistent field (SA-CASSCF) gradients employing density fitting (DF) is presented. The DF allows a reduction both in scaling and prefactors of the different steps involved. The performance of the algorithm is demonstrated on a set of molecules ranging up to an iron-Heme b complex which with its 79 atoms and 811 basis functions is to our knowledge the largest SA-CASSCF gradient computed. For smaller systems where the conventional code could still be used as a reference, both the linear response calculation and the gradient formation showed a clear timing reduction and the overall cost of a geometry optimization is typically reduced by more than one order of magnitude while the accuracy loss is negligible.

National Category
Theoretical Chemistry
Research subject
Chemistry with specialization in Quantum Chemistry
Identifiers
urn:nbn:se:uu:diva-243572 (URN)10.1063/1.4927228 (DOI)000358929100016 ()26233110 (PubMedID)
Funder
Swedish Research CouncileSSENCE - An eScience Collaboration
Available from: 2015-02-10 Created: 2015-02-10 Last updated: 2017-12-04Bibliographically approved
4. Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states
Open this publication in new window or tab >>Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states
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2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 12, 124116Article in journal (Refereed) Published
Abstract [en]

The metal L-edge (2p -> 3d) X-ray absorption spectra are affected by a number of different interactions: electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging. The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints. Here, the applicability of the method is tested in detail by simulating three ferric (3d(5)) model systems with well-known electronic structure, viz., atomic Fe3+, high-spin [FeCl6](3-) with ligand donor bonding, and low-spin [Fe(CN)(6)](3-) that also has metal backbonding. For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model. It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations. The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features. A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-236075 (URN)10.1063/1.4896373 (DOI)000342844100021 ()25273421 (PubMedID)
Note

Correction in: Journal of Chemical Physics, vol. 141, issue 4, article number: 149905, DOI: 10.1063/1.4908043 ISI: 000349847000064

Available from: 2014-11-12 Created: 2014-11-12 Last updated: 2017-12-05Bibliographically approved
5. Cost and stability core restricted active space calculations of L-edge X-ray absorption spectra.
Open this publication in new window or tab >>Cost and stability core restricted active space calculations of L-edge X-ray absorption spectra.
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(English)Manuscript (preprint) (Other academic)
National Category
Theoretical Chemistry
Research subject
Chemistry with specialization in Quantum Chemistry
Identifiers
urn:nbn:se:uu:diva-243570 (URN)
Available from: 2015-02-10 Created: 2015-02-10 Last updated: 2015-03-13
6. Simulations of iron K pre-edge X-ray absorption spectra using the core restricted active space method
Open this publication in new window or tab >>Simulations of iron K pre-edge X-ray absorption spectra using the core restricted active space method
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 4, 3250-3259 p.Article in journal (Refereed) Published
Abstract [en]

The intensities and relative energies of metal K pre-edge features are sensitive to both geometric and electronic structures. With the possibility to collect high-resolution spectral data it is important to find theoretical methods that include all important spectral effects: ligand-field splitting, multiplet structures, 3d-4p orbital hybridization, and charge-transfer excitations. Here the restricted active space (RAS) method is used for the first time to calculate metal K pre-edge spectra of open-shell systems, and its performance is tested against on six iron complexes: [FeCl6](n-), [FeCl4](n-), and [Fe(CN)(6)](n-) in ferrous and ferric oxidation states. The method gives good descriptions of the spectral shapes for all six systems. The mean absolute deviation for the relative energies of different peaks is only 0.1 eV. For the two systems that lack centrosymmetry [FeCl4](2-/1-), the ratios between dipole and quadrupole intensity contributions are reproduced with an error of 10%, which leads to good descriptions of the integrated pre-edge intensities. To gain further chemical insight, the origins of the pre-edge features have been analyzed with a chemically intuitive molecular orbital picture that serves as a bridge between the spectra and the electronic structures. The pre-edges contain information about both ligand-field strengths and orbital covalencies, which can be understood by analyzing the RAS wavefunction. The RAS method can thus be used to predict and rationalize the effects of changes in both the oxidation state and ligand environment in a number of hard X-ray studies of small and medium-sized molecular systems.

National Category
Theoretical Chemistry
Research subject
Chemistry with specialization in Quantum Chemistry
Identifiers
urn:nbn:se:uu:diva-243571 (URN)10.1039/c5cp07487h (DOI)000369506000108 ()26742851 (PubMedID)
Funder
Marcus and Amalia Wallenberg FoundationSwedish Research CouncilCarl Tryggers foundation Knut and Alice Wallenberg Foundation, KAW-2013.0020Swedish National Infrastructure for Computing (SNIC), snic2013-1-317Swedish National Infrastructure for Computing (SNIC), snic2014-5-36
Available from: 2015-02-10 Created: 2015-02-10 Last updated: 2017-12-04Bibliographically approved

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