Application of an equation-of-motion coupled cluster method including higher-order corrections to potential energy surfaces of radicals

A method proposed some time ago to correct for residual correlation effects within the framework of equation-of-motion coupled-cluster theory for ioni zed states in the singles and doubles approximation (EOMIP-CCSD) is discuss ed, and a minor modification is suggested that provides a more balanced the oretical treatment. The resulting approach is used to calculate properties of diatomic radicals, where comparison with experiment and other high-level calculations is straightforward. In addition, two electronic states of NO2 and the ground state of the cyclic C3H molecule are studied, cases in whic h symmetry-breaking phenomena play an important role. Results indicate that EOMIP-CCSD* generally provides a systematic improvement relative to the un corrected EOMIP-CCSD approach for predicting properties of radicals. (C) 19 99 American Institute of Physics. [S0021-9606(99)30142-2].