Path-integral simulation of finite-temperature properties of systems involving multiple, coupled electronic states

It is shown here how imaginary-time, path-integral methods can be easily ex tended to the determination of thermal properties of systems in which nucle ar motion occurs simultaneously on several potential energy surfaces associ ated with multiple low-lying electronic states. To do so requires exponenti ation of the full matrix of the potential in the diabatic basis., rather th an the scalar potential which occurs when motion is limited to a single pot ential energy surface. In the adiabatic approximation, the partition functi on and subsequent thermal averages are sums of those associated, separately , with each adiabatic electronic state. (C) 2001 Published by Elsevier Scie nce B.V.