Perturbative triples correction for the equation-of-motion coupled-cluster wave functions with single and double substitutions for ionized states: Theory, implementation, and examples

A noniterative N-6 triples energy correction is presented for the equation-of-motion coupled-cluster method with single and double substitutions for ionized states (EOM-IP-CCSD). The correction, which is size intensive, is derived using a second-order Rayleigh-Schrodinger perturbative treatment and is similar to the approach of Stanton and Gauss [Theor. Chim. Acta 93, 303 (1996)]. In the present implementation, only the target EOM-IP states are corrected, and the reference state is described by CCSD; the method is therefore more useful for the study of the target states themselves than ionization potentials. The performance of the correction, which demonstrates the caveat above, is demonstrated by applications to singlet methylene, BNB-, nitrogen, carbon monoxide, acetylene, benzene, thymine, and adenine. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3231133]