New relativistic ANO basis sets for actinide atoms

New basis sets of the atomic natural orbital (ANO) type have been developed for the actinide atoms Ac–Cm. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in a electric field. Scalar relativistic effects are included through the use of a Douglas–Kroll–Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The lowest multiplet levels have been computed. These calculations include spin–orbit coupling using a variation-perturbation approach. The atomic polarizability of the spherically symmetric americium atom has been computed to be 116 au3.