A new electronic structure method for doublet states: Configurationinteraction in the space of ionized 1 h and 2h1 p determinants

摘要

An implementation of gradient and energy calculations for configuration interaction variant ofequation-of-motion coupled cluster with single and double substitutions for ionization potentials(EOM-IP-CCSD) is reported. The method (termed IP-CISD) treats the ground and excited doubletelectronic states of an N-electron system as ionizing excitations from a closed-shell N+ 1-electronreference state. The method is naturally spin adapted, variational, and size intensive. Thecomputational scaling is N~5,in contrast with the N~6scaling of EOM-IP-CCSD. The performanceand capabilities of the new approach are demonstrated by application to the uracil cation and waterand benzene dimer cations by benchmarking IP-CISD against more accurate IP-CCSD. Theequilibrium geometries, especially relative differences between different ionized states, are wellreproduced. The average absolute errors and the standard deviations averaged for all bond lengthsin all electronic states (58 values in total) are 0.014 and 0.007 A, respectively. IP-CISDsystematically underestimates intramolecular distances and overestimates intermolecular ones,because of the underlying uncorrelated Hartree–Fock reference wave function. The IP-CISDexcitation energies of the cations are of a semiquantitative value only, showing maximum errors of0.35 eV relative to EOM-IP-CCSD. Trends in properties such as dipole moments, transition dipoles,and charge distributions are well reproduced by IP-CISD.