Exploring Biorthonormal Transformations of Pair-Correlation Functions in Atomic Structure Variational Calculations

TL;DR

This paper introduces a novel method for atomic structure calculations that optimizes independent pair-correlation functions with biorthonormal transformations, enabling more accurate core correlation modeling and improved energy estimates.

Contribution

The paper proposes a new variational approach using independent MCHF pair-correlation functions with biorthonormal transformations, enhancing core correlation treatment in atomic calculations.

Findings

Lower total energies for beryllium compared to traditional methods

Effective accounting for deep core correlation effects

Use of targeted, localized orbital sets for each PCF

Abstract

Multiconfiguration expansions frequently target valence correlation and correlation between valence electrons and the outermost core electrons. Correlation within the core is often neglected. A large orbital basis is needed to saturate both the valence and core-valence correlation effects. This in turn leads to huge numbers of CSFs, many of which are unimportant. To avoid the problems inherent to the use of a single common orthonormal orbital basis for all correlation effects in the MCHF method, we propose to optimize independent MCHF pair-correlation functions (PCFs), bringing their own orthonormal one-electron basis. Each PCF is generated by allowing single- and double- excitations from a multireference (MR) function. This computational scheme has the advantage of using targeted and optimally localized orbital sets for each PCF. These pair-correlation functions are coupled together and with each component of the MR space through a low dimension generalized eigenvalue problem. Nonorthogonal orbital sets being involved, the interaction and overlap matrices are built using biorthonormal transformation of the coupled basis sets followed by a counter-transformation of the PCF expansions. Applied to the ground state of beryllium, the new method gives total energies that are lower than the ones from traditional CAS-MCHF calculations using large orbital active sets. It is fair to say that we now have the possibility to account for, in a balanced way, correlation deep down in the atomic core in variational calculations.