Excited states of open-shell molecules, play major roles in combustion and atmospheric chemistry, as well as photochemistry. However, theoretical simulations of open-shell excited states pose challenges in describing orbital relaxation, electron correlation, and charge localization effects. Additionally, excited state calculations of open-shell systems may suffer from spin contamination (SC) error. Algebraic diagrammatic construction theory (ADC) is a perturbative post Hartree–Fock (HF) method that balances the accuracy and computational cost to simulate molecular excited states. In this talk, I will quantify the SC error in ADC simulations of charged and neutral excitations in open-shell systems and demonstrate how to reduce SC by using different reference wavefunctions. These reference wavefunctions include orbital optimized Møller–Plesset perturbation theory (OMP) and restricted open-shell HF (ROHF). I will show that combining ADC with the ROHF and OMP reference wavefunctions reduces the SC error and significantly improves the accuracy of ADC for open-shell molecular excited states.

Quantifying and reducing spin contamination in algebraic diagrammatic construction theory

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