title: Assessment of low-scaling approximations to the equation of motion coupled-cluster singles and doubles equations.
abstract: Methods for fast and reliable computation of electronic excitation energies are in short supply, and little is known about their systematic performance. This work reports a comparison of several low-scaling approximations to the equation of motion coupled cluster singles and doubles (EOM-CCSD) and linear-response coupled cluster singles and doubles (LR-CCSD) equations with other single reference methods for computing the vertical electronic transition energies of 11 small organic molecules. The methods, including second order equation-of-motion many-body perturbation theory (EOM-MBPT2) and its partitioned variant, are compared to several valence and Rydberg singlet states. We find that the EOM-MBPT2 method was rarely more than a tenth of an eV from EOM-CCSD calculated energies, yet demonstrates a performance gain of nearly 30%. The partitioned equation-of-motion approach, P-EOM-MBPT2, which is an order of magnitude faster than EOM-CCSD, outperforms the CIS(D) and CC2 in the description of Rydberg states. CC2, on the other hand, excels at describing valence states where P-EOM-MBPT2 does not. The difference between the CC2 and P-EOM-MBPT2 can ultimately be traced back to how each method approximates EOM-CCSD and LR-CCSD. The results suggest that CC2 and P-EOM-MBPT2 are complementary: CC2 is best suited for the description of valence states while P-EOM-MBPT2 proves to be a superior O(N(5)) method for the description of Rydberg states.
authors.affiliation: Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Goings
authors.affiliation: Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
authors.forename: Marco
authors.initials: M
authors.lastname: Caricato
authors.affiliation: Gaussian, Inc., 340 Quinnipiac St, Bldg 40, Wallingford, Connecticut 06492, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Frisch
authors.affiliation: Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Xiaosong
authors.initials: X
authors.lastname: Li