HIGH SECTORS IN THE FOCK SPACE COUPLED-CLUSTER METHOD

The coupled-cluster method with single and double excitations (CCSD or full SUB2) is applied to systems with n electrons outside a closed shell, n less-than-or-equal-to 6. The CCSDT1 scheme, which includes triple excitations approximately, is also employed. The systems of interest are the F atom and its ions. F5+ is the reference state, and the ionization potentials and excitation energies involving the 2p and 3s electrons are calculated. The CCSD results are good for n = 1 and 2, but deteriorate with increasing n. The CCSDT1 scheme gives satisfactory results in the n = 3 and 4 sectors, but overestimates the effects for higher n.