Natural transition orbitals for complex two‐component excited state calculations

While the natural transition orbital (NTO) method has allowed electronic excitations from time‐dependent Hartree‐Fock and density functional theory to be viewed in a traditional orbital picture, the extension to multicomponent molecular orbitals such as those used in relativistic two‐component methods or generalized Hartree‐Fock (GHF) or generalized Kohn‐Sham (GKS) is less straightforward due to mixing of spin‐components and the inherent inclusion of spin‐flip transitions in time‐dependent GHF/GKS. An extension of single‐component NTOs to the two‐component framework is presented, in addition to a brief discussion of the practical aspects of visualizing two‐component complex orbitals. Unlike the single‐component analog, the method explicitly describes the spin and frequently obtains solutions with several significant orbital pairs. The method is presented using calculations on a mercury atom and a CrO 2 Cl 2 complex.