Analytic gradients for EOM-DEA-CCSD and EOM-DIP-CCSD: Theory, implementation, and application to diradicals

T. Zhao and A.I. Krylov
J. Chem. Phys. , submitted (2025)

Equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) provides a robust framework for describing a wide range of electronically excited and open-shell states. Among its various formulations, the double electron-attachment (EOM-DEA-CCSD) and double ionization potential (EOM-DIP-CCSD) methods are particularly effective for treating diradicals and other types of open-shell species. To enable accurate geometry optimizations and property calculations, we present the derivation and implementation of analytic nuclear gradients for EOM-DEA-CCSD and EOM-DIP-CCSD. These new capabilities were illustrated by calculations of singlet-triplet gaps in benzyne diradicals as well as characterization of molecules relevant to quantum information science. We further extended the framework to include spin–orbit coupling (SOC) calculations for EOM-DEA-CCSD states, allowing calculations of intensity borrowing and intersystem crossings.

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