Hannover 2020 – scientific programme

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MO: Fachverband Molekülphysik

MO 17: Theory

MO 17.7: Talk

Thursday, March 12, 2020, 15:30–15:45, f142

Is nuclear motion classical? The success of the Maxwell-Boltzmann distribution — •Irmgard Frank — Theoretical Chemistry, Leibniz University of Hannover, Callinstr. 3A, 30167 Hannover

The Born-Oppenheimer approximation can be replaced by a better approximation in a very simple way. The consideration starts from the notion that it is problematic to describe the nuclear motion and the electronic cloud in a similar way, hoping that the Schrödinger equation will generate meaningful results for both of these two completely different types of objects. As the result of the simulation of a chemical reaction one wants to obtain well localized nuclei which clearly define the product state, not a nuclear cloud. The only consistent way to achieve this is by treating the motion of the nuclei classically right from the beginning. The immediate result is ab-initio molecular dynamics (AIMD). Movies of chemical reactions generated with AIMD show clearly that this approach is working well at normal energies. Tunneling is electronic tunneling in every case. Chemical reactions are explained by outliers of the classical Maxwell-Boltzmann velocity distribution. The development of this distribution is guaranteed in a molecular dynamics run as it is the most likely distribution and defines the temperature of the system. In the present study we investigate some critical cases, namely the ammonia inversion and the heat capacities of hydrogen and water.