Multiconfigurational Quantum Mechanics (QM) for Heavy Element Compounds

Abstract

One frequently used quantum chemical approach for the studies of transition metal and other heavy element compounds is the complete active space self consistent field (CASSCF) method in combination with multiconfigurational second‐order perturbation theory (CASPT2). Here, we describe this method and some of its typical applications. The basic idea is to construct a wave function that gives a qualitatively correct description of the electronic structure already at the lowest level of theory. This should be possible for all sorts of electronic states independent of spin multiplicity and excitation level. Strong mixing of configurations makes it necessary to use a multiconfigurational approach in such cases. It should also be possible to use for all atoms of the periodic system. The CASSCF wave function fulfills, in principle, this requirement because it is full configuration interaction (CI), albeit in a limited space of active orbitals. The addition of dynamic electron correlation is necessary to give quantitative results. The suggested solution is to compute this energy using second‐order perturbation theory (CASPT2) because it is relatively simple and allows applications to a wide variety of systems and many electrons. Relativistic corrections are included using a Douglas‐Kroll‐Hess Hamiltonian. The method is scrutinized and illustrated by some applications in transition metal and actinide chemistry.