ANALYTIC ENERGY DERIVATIVES FOR IONIZED STATES DESCRIBED BY THE EQUATION-OF-MOTION COUPLED-CLUSTER METHOD

The theory for analytic energy derivatives of excited electronic state s described by the equation-of-motion coupled cluster (EOM-CC) method has been generalized to treat cases in which reference and final state s differ in the number of electrons. While this work specializes to th e sector of Fock space that corresponds to ionization of the reference , the approach can be trivially modified for electron attached final s tates. Unlike traditional coupled cluster methods that are based on si ngle determinant reference functions, several electronic configuration s are treated in a balanced way by EOM-CC. Therefore, this quantum che mical approach is appropriate for problems that involve important nond ynamic electron correlation effects. Furthermore, a fully spin adapted treatment of doublet electronic states is guaranteed when a spin rest ricted closed shell reference state is used-a desirable feature that i s not easily achieved in standard coupled cluster approaches. The effi cient implementation of analytic gradients reported here allows this v ariant of EOM-CC theory to be routinely applied to multidimensional po tential energy surfaces for the first time. Use of the method is illus trated by an investigation of the formyloxyl radical (HCOO), which suf fers from notorious symmetry breaking effects.