A natural decay of mixing algorithm for non-Born-Oppenheimer trajectories

We present a new method called the natural decay of mixing (NDM) method for introducing decoherence effects into the semiclassical Ehrenfest self-cons istent potential method. The NDM method is similar in spirit to two recentl y developed methods, the continuous surface switching (CSS) and continuous surface switching II (CSS2) methods, but, like the pure semiclassical Ehren fest method, it involves only a single variable that serves as both the wei ght of an electronic state and its electronic population. We demonstrate ho w this allows the NDM method to be applied to systems where the CSS and CSS 2 methods cannot be applied, and also to cases where the CSS and CSS2 metho ds would be prohibitively expensive. The method is tested for electronicall y nonadiabatic processes, both reactive and nonreactive, and in a wider con text it contributes to the rapidly blossoming fields of quantum measurement and hybrid quantum/classical algorithms for the dynamics of complex system s. (C) 2001 American Institute of Physics.