Spin-adapted open-shell time-dependent density functional theory: towards a simple and accurate method for spin-flip-down excitations

TL;DR

This paper introduces XSF-TDA, a new spin-adapted method for spin-flip-down excitations in TD-DFT, addressing spin contamination issues and demonstrating improved accuracy over existing methods in prototype systems.

Contribution

The paper develops XSF-TDA, a simple and accurate spin-adapted approach for spin-flip-down excitations, improving upon previous SF-TD-DFT methods.

Findings

XSF-TDA outperforms USF-TDA and MRSF-TDA in prototype tests.

Demonstrates better description of bond breaking and inverted singlet-triplet gaps.

Addresses spin contamination effectively in spin-flip TD-DFT.

Abstract

A major challenge in using spin-flip time-dependent density functional theory (SF-TD-DFT) for spin-flip-down excitations is the presence of spin contamination. While several improved methods have been developed in the past, a simple and accurate method remains elusive. Here, based on our previous development on spin-adapted open-shell TD-DFT for spin-conserving excitations (X-TD-DFT) [Z. Li and W. Liu, J. Chem. Phys. 135, 194106 (2011)], we introduce a method termed as XSF-TDA for modeling spin-flip-down excitations, and provide an in-depth comparison of different methodologies for mitigating spin contamination in SF-TDA. Pilot applications to prototype systems demonstrate the promise of XSF-TDA over existing SF-TDA methods, including unrestricted SF-TDA (USF-TDA) and mixed-reference SF-TDA (MRSF-TDA), in describing bond breakings and inverted singlet-triplet gap systems.