THE ORIGIN OF DIFFERENCES BETWEEN COUPLED-CLUSTER THEORY AND QUADRATIC CONFIGURATION-INTERACTION FOR EXCITED-STATES

By employing time-dependent linear response theory, the ground state quadratic configuration interaction method with single and double substitutions (QCISD) can be generalized to excited states. However, the excited state QCISD method is numerically inferior to the analogous coupled cluster (CCSD) excited state method, although its formal properties of size consistency and exactness for two electrons are the same. Therefore a modified QCISD method which includes an additional quadratic operator term in the doubles equations, as in the CPMET-C approximation, is investigated. It is shown by calculations on a variety of small molecules to correct substantially the deficiencies of QCISD for excited states, yielding results that are close to full CCSD theory. This CPMET-C based excited state method is size-consistent, but unlike CCSD and QCISD, is not exact for separated pairs of electrons.