Short-time dynamics through conical intersections in macrosystems.

The short-time dynamics through a conical intersection of a macrosystem with a vast number of nuclear degrees of freedom (modes) is investigated. For convenience, the macrosystem is decomposed into a system carrying a few modes and a "bath." By transforming the bath modes to new ones, it is shown that only three effective bath modes contribute to the conical intersection. They govern--together with the system's modes--the short-time dynamics of the macrosystem. The remaining bath modes do not directly couple the electronic states and become relevant at longer times. An extensive numerical example is presented.