title: Correlation correction to configuration interaction singles from coupled cluster perturbation theory.
abstract: A new state specific correlation correction to configuration interaction singles (CIS) excitation energies is presented using coupled cluster perturbation theory (CCPT). General expressions for CIS-CCPT are derived and expanded explicitly to first-order in the wavefunction and second-order in the energy. By virtue of the nature of CCPT this method is a priori size extensive and incorporates infinite-order effects into the wavefunction. This results in a balanced singles space excited state theory that at second-order is an improvement over the ubiquitous CIS(D) method and comparable in quality to equation of motion coupled cluster (EOM-CC). A modest test set composed of the first four excited states from nine small organic molecules was used to quantify the accuracy and consistency of the CIS-CCPT2 excitation energies and density of states. We find that CIS-CCPT2 has a standard deviation error of 0.18 eV for excitation energies and 0.14 eV for density of states compared to EOM-CC, a factor of two better than CIS(D) with a significant reduction in the maximum deviation as well.
authors: Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA.
authors: Jason N
authors: JN
authors: Byrd
authors: Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA.
authors: Victor F
authors: VF
authors: Lotrich
authors: Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA.
authors: Rodney J
authors: RJ
authors: Bartlett