<i>Ab Initio</i> Valence Bond Molecular Dynamics: A Study of S_N2 Reaction Mechanisms

In this paper, a molecular dynamics (MD) approach based on <i>ab initio</i> classical valence bond (VB) theory, referred to as AIVBMD, is presented. To validate AIVBMD, a novel algorithm that enables efficient computation of energy gradients based on nonorthogonal orbitals is introduced. Taking the gas-phase S_N2 reaction as an example, a compact VB wave function gives reasonable accuracy with only 27 VB structures, compared to the full active space of 5292 VB structures. Furthermore, AIVBMD provides intuitive chemical insights into the reaction process, detailing the breaking and formation of chemical bonds, thereby elucidating the reaction mechanism. In summary, as the first attempt at the <i>ab initio</i> classical VB method-based MD approach, this paper demonstrates that VB theory offers a novel perspective and significant potential for investigating chemical reaction dynamics and mechanisms.