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We study the ultrafast demagnetization dynamics of 𝐿⁢𝑛⁢Rh₂⁢Si₂(𝐿⁢𝑛=Pr, Nd, Sm, Gd, Tb, Dy, Ho) antiferromagnets after excitation by a laser pulse, using a combination of density functional theory and atomistic spin and spin-lattice dynamics simulations. In the first step, we calculate the Heisenberg interactions using the magnetic force theorem and compare two approaches, where the 4⁢𝑓 states of the rare earths are treated as frozen core states or as valence states with added correlation corrections. We find marked quantitative differences in terms of predicted Curie temperature for most of the systems, especially for those with a large orbital moment of the rare-earth cations. This can be attributed to the importance of indirect interactions of the 4⁢𝑓 states through the Si states, which depends on the binding energy of the 4⁢𝑓 states and coexists with Ruderman-Kittel-Kasuya-Yosida–type interactions mediated by the conduction states. However, qualitatively both approaches agree in terms of the predicted antiferromagnetic ordering at low temperature, which is in line with previous experiments. In the second step, the atomistic dynamics simulations are used in combination with a heat-conserving two-temperature model, which allows for the calculation of spin and electronic temperatures during the magnetization dynamics simulations. Our simulations demonstrate that despite quite different demagnetization times, magnetization dynamics of all studied 𝐿⁢𝑛⁢Rh₂⁢Si₂ antiferromagnets exhibit similar two-step behavior, in particular the first fast drop followed by slower demagnetization. In addition, we observe that the demagnetization amplitude depends linearly on laser fluence, for low fluences, something that is also in agreement with experimental observations. We also investigate the impact of lattice dynamics on ultrafast demagnetization using coupled atomistic spin-lattice dynamics simulations and the heat-conserving three-temperature model, which confirm the linear dependence of magnetization on laser fluence. The microscopic mechanisms behind these behaviors are investigated in detail.