Relativistic Complete Active Space Self-Consistent-Field Method with a Hierarchy of Exact Two-Component Hamiltonians.

The development of a novel exact two-component (X2C) scheme with the inclusion of the picture-change correction for the fluctuation potential, the "X2Ccorr" scheme, is reported, hereby establishing a hierarchy of X2C schemes with systematic improvement for the treatment of relativistic two-electron contributions. Using benchmark X2C complete active space self-consistent-field (CASSCF) calculations for zero-field splittings in chalcogen diatomics, the contributions of two-electron spin-orbit coupling, electron spin-spin coupling, and quantum electrodynamics are carefully analyzed. The capability of the new Cholesky decomposition-based implementation for the relativistic two-component CASSCF method using super-configuration-interaction algorithms is further demonstrated with calculations for the low-lying electronic states of neodymium aqua-ions with up to the second coordination shells.