A new electronic structure method for doublet states: configuration interaction in the space of ionized 1h and 2h1p determinants.

An implementation of gradient and energy calculations for configuration interaction variant of equation-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 doublet electronic states of an N-electron system as ionizing excitations from a closed-shell N+1-electron reference state. The method is naturally spin adapted, variational, and size intensive. The computational scaling is N(5), in contrast with the N(6) scaling of EOM-IP-CCSD. The performance and capabilities of the new approach are demonstrated by application to the uracil cation and water and benzene dimer cations by benchmarking IP-CISD against more accurate IP-CCSD. The equilibrium geometries, especially relative differences between different ionized states, are well reproduced. The average absolute errors and the standard deviations averaged for all bond lengths in all electronic states (58 values in total) are 0.014 and 0.007 A, respectively. IP-CISD systematically underestimates intramolecular distances and overestimates intermolecular ones, because of the underlying uncorrelated Hartree-Fock reference wave function. The IP-CISD excitation energies of the cations are of a semiquantitative value only, showing maximum errors of 0.35 eV relative to EOM-IP-CCSD. Trends in properties such as dipole moments, transition dipoles, and charge distributions are well reproduced by IP-CISD.