Quasidiabatic states described by coupled-cluster theory.

In an attempt to expand the utility of the model Hamiltonian technique developed by Koppel, Domcke, and Cederbaum (KDC) [Adv. Chem. Phys. 57, 59 (1984)], an ansatz for quasidiabatic wave functions is introduced in the framework of equation-of-motion coupled-cluster (EOM-CC) theory. Based on the ansatz, the theory for the analytic first derivative of the off-diagonal element of the quasidiabatic potential matrix is developed by extending the theory for the analytic gradient of the EOM-CC energy. This analytic derivative is implemented for EOM-CCSD (singles and doubles approximation) calculations of radicals subject to pseudo-Jahn-Teller and Jahn-Teller interactions. Its applicability in construction of the KDC quasidiabatic model potential is discussed.