title: Equation-of-motion coupled-cluster methods for ionized states with an approximate treatment of triple excitations.
abstract: The accuracy of geometries and harmonic vibrational frequencies is evaluated for two equation-of-motion ionization potential coupled-cluster methods including CC3 and CCSDT-3 triples corrections. The first two Sigma states and first Pi state of the N2 +, CO+, CN, and BO diatomic radicals are studied. The calculations show a tendency for the CC3 variant to overestimate the bond lengths and to underestimate the vibrational frequencies, while the CCSDT-3 variant seems to be more reliable. It is also demonstrated that the accuracy of such methods is comparable to sophisticated traditional multireference approaches and the full configuration interaction method.
authors.affiliation: Department of Chemistry and Biochemistry, Institute for Theoretical Chemistry, University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Yannick J
authors.initials: YJ
authors.lastname: Bomble
authors.forename: Jamal C
authors.initials: JC
authors.lastname: Saeh
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.forename: Péter G
authors.initials: PG
authors.lastname: Szalay
authors.forename: Mihály
authors.initials: M
authors.lastname: Kállay
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gauss