Second-order generalized unrestricted Møller–Plesset perturbation theory for the spin–orbit part of zero-field splitting tensors

Novel zero-field splitting (ZFS) formula is developed to calculate the spin–orbit contribution (DSO) to the ZFS tensor by using finite-perturbation theory, i.e., two-component self-consistent-field procedures with second-order Douglas–Kroll–Hess Hamiltonian. The ZFS formula is extended to second-order generalized unrestricted Møller–Plesset perturbation theory. For triplet diatomic molecules, several fundamental contributions to the DSO tensor are investigated. The electron-correlation effects were relatively large (more than 23%) and were very significant (more than 56%) especially for SeS, Se2, and Te2.