NONADIABATIC TRANSITIONS: WhatWe Learned from Old Masters and How MuchWe Owe Them

This chapter discusses the impact of two-state models of nonadiabatic coupling by Landau, Zener, St¨uckelberg, Rosen, and Teller on the current theory of nonadiabatic interaction. In particular, the idea of analytical continuation of classical dynamical variables into complex-valued phase space and time is emphasized. The development of the basic models over the past 30 years has provided us with a useful means of investigating the nonadiabatic molecular dynamics; this is illustrated by the