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 demonstrated for third row atoms. The results show that the two methods give results of the same accuracy, with the exception of first row atoms with less than five 3d-electrons. The CASPT2 method here overestimates the effect of 3p correlation with about 0.1 eV.