
 The BornOppenheimer Approximation 
  Of course, the problem cannot actually be solved exactly, so approximation methods are needed and it is in the development of approximation methods and an analysis of their accuracy that quantum chemistry is focused. 
  We first invoke the BornOppenheimer approximation by recognizing that, in a dynamical sense, there is a strong separation of time scales between the electronic and nuclear motion, since the electrons are lighter than the nuclei by three orders of magnitude. 
  This, of course, is an approximation, since there could be nonadiabatic effects that do not allow the electrons to follow in this ``instantaneous'' manner, however, in many systems, the adiabatic separation between electrons and nuclei is an excellent approximation. 
 www.nyu.edu /classes/tuckerman/quant.mech/lectures/lecture_10/node1.html (1048 words) 
