Nuclear energy molecular change effects

In molecular dynamics simulations, quantum mechanics is used to calculate the energy of the quantum system but the motion of the whole system is treated classically. Taking account of the quantum character of the motion does not invalidate eqn (16.1), but it gives rise to nuclear quantum mechanical (NQM) effects which change AG and the transmission coefficient k. Zero point energy effects contribute to AG while dynamical effects such as tunnelling and transitions to excited vibrational states contribute to k. Various approaches have been proposed to evaluate these contributions. 

Following a description of femtosecond lasers, the remainder of this chapter concentrates on the nuclear dynamics of molecules exposed to ultrafast laser radiation rather than electronic effects, in order to try to understand how molecules fragment and collide on a femtosecond time scale. Of special interest in molecular physics are the critical, intermediate stages of the overall time evolution, where the rapidly changing forces within ephemeral molecular configurations govern the flow of energy and matter. 

Changes in molecular properties on isotopic substitution are purely mass effects in that the force constants of the molecule are unaltered. Ground state geometry is unchanged and decompositions of the iso-topically distinct molecules occur over identical potential energy surfaces. These conditions hold to the extent that the Bom—Oppenheimer approximation separating nuclear and electronic motions is valid. Vibrational frequencies and moments of inertia are modified by isotopic... 

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