Relativistic Complete Active Space Self-consistent-Field Method with a Hierarchy of Exact Two-Component Hamiltonians

TL;DR

This paper introduces a new hierarchy of exact two-component (X2C) Hamiltonians for relativistic quantum chemistry, improving the treatment of two-electron effects in CASSCF calculations and demonstrating its effectiveness on various benchmark systems.

Contribution

It presents a novel X2C scheme with picture-change correction, establishing a systematic hierarchy for better relativistic two-electron treatment in CASSCF methods.

Findings

Accurate zero-field splitting calculations for chalcogen diatomics.

Effective implementation for complex systems like neodymium aqua-ions.

Systematic improvement over previous X2C schemes.

Abstract

The development of a novel exact two-component (X2C) scheme with the inclusion of the picture-change correction for the fluctuation potential, the X2Ccorr scheme, is reported, hereby establishing a hierarchy of X2C schemes with systematic improvement for the treatments of relativistic two-electron contributions. Using benchmark X2C complete active space self-consistent-field (CASSCF) calculations for zero-field splittings in chalcogen diatomics, the contributions of two-electron spin-orbit coupling, electron spin-spin coupling, and quantum electrodynamics are carefully analyzed. The capability of the new Cholesky decomposition-based implementation for relativistic two-component CASSCF method using super-configuration-interaction algorithms is further demonstrated with calculations for the low-lying electronic states of neodymium aqua-ions with up to the second coordination shells.