ATOMIC NATURAL ORBITAL (ANO) BASIS-SETS FOR QUANTUM-CHEMICAL CALCULATIONS

This chapter introduces an approach to basis set design— that is, to use physically motivated atomic orbitals as basis functions for molecular calculations. The fact that the AOs are expanded as fixed contractions of Gaussian functions is a physically irrelevant computational device. For correlated calculations, the natural orbitals derived from correlated atomic wave functions prove to be a compact, computationally efficient, and physically meaningful set of basis functions. The efficient evaluation of AO integrals over such basis sets imposes certain computational requirements on an integral code. The chapter discusses how these requirements are implemented in evaluation of two-electron integrals over a generally contracted Gaussian basis set in the molecule integral program. The chapter also discusses how ANO basis sets are effectively able to exhaust the capabilities of the underlying Gaussian expansion basis— that is, to minimize the contraction error. Finally, the chapter describes several applications of quantum chemistry to problems in which the use of ANO basis sets is significant in obtaining the required high accuracy. © 1991, Elsevier Inc. All rights reserved.