The Time-Dependent Variational Principal in Quantum Mechanics and Its Application

This account of the time-dependent variational principle is presented in memory of Per-Olov Lowdin on the occasion of the centenary of his birth. The material presented here has been published as part of a book chapter, 1 and is reintroduced here in recognition of Lowdin's interest in this topic. 2 Also the importance of the use of coherent state parameters as functions of time is emphasized, as well as the connection to the electron nuclear dynamics (END) theory.