Infinite order relaxation effects for core ionization energies with a variational coupled cluster ansatz

Direct core ionization energies are calculated with a new variational coupled cluster technique. Direct double core ionization energies for orthoaminophenol are also calculated. In this method, the reference function is the ground state N-electron Hartree–Fock determinant, and effects of orbital relaxation are summed to infinite order. This method reproduces the Δ E SCF result exactly by changing occupation. An approximation that keeps the occupation constant is demonstrated. The average absolute error for single ionizations using correlation-consistent double and triple-ζ basis sets with this approximation are 0.007 and 0.008 eV, respectively. For double ionizations it is 0.23 eV, under 1% of the relaxation energy.