Excitation energies for transition metal atoms - A comparison between coupled cluster methods and second-order perturbation theory
Coupled cluster methods are compared to multiconfigurational second-order perturbation theory in a study of the high spin d(x)s(2) -> d(x+1)s(1) excitations in first, second, and third row transition metals. Large basis sets of the atomic natural orbital type are used. Scalar relativistic effects are included using the Douglas-Kroll-Hess Hamiltonian. The effect of spin-orbit coupling is demonstrat