Finite-field method calculations of molecular polarisabilities. I. Theoretical basis and limitations of SCF and Galerkin treatments

Abstract

In calculation of polarisabilities by the finite-field method, a number of problems arise which require critical examination. In this paper some of the most important of these are considered for calculations at the SCF level and illustrated by results obtained for the polarisabilities of LiH, Li 2 and N 2. The special basis-set requirements for obtaining accurate molecular polarisabilities are discussed. The problem of achieving basis-set convergence is also considered. An approximation at the SCF level — the weak formulation or Galerkin method — is introduced to reduce the computational expense resulting from the use of large basis sets. It is found that this method gives results comparable with those of the full SCF treatment with considerable savings of time. Correlation effects will be discussed in part II.