Second quantisation for unrestricted references: formalism and quasi-spin-adaptation of excitation and spin-flip operators

Abstract

A second quantised formulation for the evaluation of spin-dependent properties in the UHF basis will be introduced. The configuration interaction singles ansatz will be used as an illustration of the practical usage of this formulation and the results will be compared to spin-restricted cases of the same ansatz. On a more tentative basis, we will also discuss the notion of quasi-spin-adaptation which ensures that the ansatz becomes spin-adapted as the unrestricted orbitals approach the restricted ones.

GRAPHICAL ABSTRACT

Keywords:

• UHF
• second quantisation
• spin adaptation
• configuration state functions

Disclosure statement

No potential conflict of interest was reported by the author.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.