Chapter 6 Two-component relativistic effective core potential calculations for molecules

Abstract

Spin-orbit and other relativistic effects important for the reliable description of valence states of atoms and molecules can be represented as two-component relativistic effective core potentials (RECPs) derived from Dirac-Coulomb Hamiltonian based all-electron calculations of atoms. Using spin-orbit RECPs, we have implemented and tested a series of two-component methods for molecular electronic structure calculations starting from a two-component Kramers' restricted Hartree-Fock (KRHF) method for the polyatomic molecules with closed-shell configurations. The KRHF method utilizes RECPs with effective one-electron spin-orbit operators at the Hartree-Fock level in a variational manner and produces molecular spinors obeying the double group symmetry. Electron correlations are treated at various levels including the coupled-cluster level of theory with and without spin-orbit interactions. Spin-orbit effects on many molecules containing sixth-row p -block elements (Tl∼Rn), transactinide d -block elements (Rf, Db, and Sg), and p -block elements (element 113∼ 118) are evaluated and discussed. In the present work, the spin-orbit effect is defined by the difference between the results of one- and two-component RECPs. The one-component RECP, which is derived by a potential average scheme, is found to be useful for describing spin-free molecular properties even for the transactinide molecules. The potential average scheme is proposed as the consistent definition of spin–orbit effects in any RECP scheme when spin–orbit effects are compared among various methods.