General contraction in four-component relativistic Hartree-Fock calculations

Abstract

General contraction is a well known means to reduce the computational effort for calculating electronic wave functions using basis set expansion techniques. In contrast to fixed contraction, optimal flexibility is available in constructing the “best” basis functions from basis set primitives. For relativistic four-component Dirac-Hartree-Fock calculations (and beyond), the variational space may consist of separate basis functions for the “Large” and “Small” component bases. The choice, and therefore also the contraction, of the Small component basis relative to the Large component basis is non-trivial. In particular in the case that “Kinetic Balance” is used to define the Small component basis relative to the Large component basis contraction imposes a severe problem, because the better the contraction of the Large component, the worse the kinetically balanced Small component counterpart becomes. Solutions to this dilemma are provided and have been implemented. Figures on reduction in the computational effort are given.