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

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 calculations for EOM-DEA-CCSD states, allowing calculations of intensity borrowing and intersystem crossings.