Introduction

Abstract

This book aims at bridging the gap between two active fields of research, i.e., quantum chemistry and solid-state theory. Soon after the development of quantum mechanics by Heisenberg [1.1], Schrödinger [1.2], Born and Jordan [1.3], Dirac [1.4], and others, the paper by Heitler and London [1.5] on the ground state of the H₂ molecule opened the way to a theoretical understanding of chemical bonding. Their work marks the beginning of quantum chemistry. The Heitler-London ansatz for the ground-state wavefunction of H₂ treats the two electrons as being strongly correlated, i.e., by excluding ionic configurations the two electrons stay completely out of each other’s way. The ground state is a singlet and has the form

$$\psi_{\rm HL}^{\rm S}({\bf r}_1, {\bf r}_2) = {1\over 2}\lbrack \phi_1({\bf r}_1)\phi_2({\bf r}_2)+\phi_2({\bf r}_1)\phi_1({\bf r}_2)\rbrack(\alpha_1\beta_2-\beta_1\alpha_2).$$

((1.0.1))