Quantifying and reducing spin contamination in algebraic diagrammatic construction theory of charged excitations

Algebraic diagrammatic construction (ADC) theory is a computationally efficient and accurate approach for simulating electronic excitations in chemical systems. However, for the simulations of excited states in molecules with unpaired electrons, the performance of ADC methods can be affected by the spin contamination in unrestricted Hartree-Fock (UHF) reference wavefunctions. In this work, we benchmark the accuracy of ADC methods for electron attachment and ionization of open-shell molecules with the UHF reference orbitals (EA/IP-ADC/UHF) and develop an approach to quantify the spin contamination in charged excited states. Following this assessment, we demonstrate that the spin contamination can be reduced by combining EA/IP-ADC with the reference orbitals from restricted open-shell Hartree-Fock (ROHF) or orbital-optimized Moller-Plesset perturbation (OMP) theories. Our numerical results demonstrate that for open-shell systems with strong spin contamination in the UHF reference, the third-order EA/IP-ADC methods with the ROHF or OMP reference orbitals are similar in accuracy to equation-of-motion coupled cluster theory with single and double excitations. Published under an exclusive license by AIP Publishing.

Automated summary

The ADC theory is an accurate and computationally efficient method for simulating electronic excitations in chemical systems. This study benchmarks the accuracy of ADC methods for electron attachment and ionization in open-shell molecules with unrestricted Hartree-Fock reference orbitals and proposes an approach to quantify the spin contamination in charged excited states. The results show that the spin contamination can be reduced by combining EA/IP-ADC with reference orbitals from restricted open-shell Hartree-Fock or orbital-optimized Moller-Plesset perturbation theories.