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 Hehler 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

$$ \psi _{{\text{HL}}}^{\text{S}}\left( {{{\text{r}}_1},{{\text{r}}_2}} \right) = \frac{1}{2}\left[ {{\phi _1}\left( {{{\text{r}}_1}} \right){\phi _2}\left( {{{\text{r}}_2}} \right) + {\phi _2}\left( {{{\text{r}}_1}} \right){\phi _1}\left( {{{\text{r}}_2}} \right)} \right]\left( {{\alpha _1}{\beta _2} - {\beta _1}{\alpha _2}} \right). $$

(1)