Ionization energies of water from PNO-CI calculations

A new configuration interaction method using a non-orthogonal set of pseudonatural orbitals (PNO-CI) is applied to the ground state of water and five states of the ionized molecule. Including only double substitutions, the computed upper bound for the ground state is 0.0631 a.u. above the experimental non-relativistic value. An approximate treatment of unlinked clusters lowers the energy by 0.0145 a.u. Thus above 90% of the total correlation energy is accounted for. The computed vertical ionization energies (IE's) agree with experiment to within 2%. An analysis of the IE's shows the causes of anomalous behavior of correlation energies and supports a suggestion for useful limited CI calculations of vertical IE's. The equilibrium geometries of the three lowest states of the ion are determined by SCF calculations and estimates for the adiabatic IE's are given.