Introduction

Abstract

As is well known, the fundamental laws of electronic and nuclear motions determining the structure and properties of molecules and crystals were revealed in the 1930s immediately after the discovery of quantum mechanics. Because of mathematical difficulties encountered in the quantum-mechanical description of polyatomic systems, several essential approximations are usually employed in the solution of the appropriate Schrödinger equation, and among them the adiabatic approximation is the most important [1.1, 2]. This approximation is based on the difference in the masses (and hence velocities) of electrons and nuclei; due to this difference, for every position of the nuclei at any instant a stationary distribution of the electrons is attained. Without the adiabatic approximation the notion of spatial structure (nuclear configuration) becomes uncertain.