Configuration interaction method based upon two-component spinor formalism and relativistic effective core potentials

Abstract

A two-component configuration interaction method called the Kramers`` restricted configuration interaction (KRCI) is implemented. The Hamiltonian employed in the KRCI method contains most of the relativistic effects including the spin-orbit term through the use of the relativistic effective core potentials (REP) and has two-component spinors as 1-electron eigenfunctions. The KRCI method is distinguished from the conventional configuration interaction method using the REP in the treatment of the spin-orbit term. The former introduces the spin-orbit term from the HF step, whereas the latter ignores the spin-orbit term at the Hartree-Fock (HF) step and includes the spin-orbit term only at the configuration integraction (CI) step. The KRCI method as a single-reference CI is useful to introduce spin-orbit effects into the qualitative calculations of excited states or the ground state properties. Test calculations for the Rn atom demonstrate that the single-reference CIS calculation using the KRCI method is better than the more conventional single-reference scheme of incorporating the spin-orbit term only at the CI level. Test calculations for the ground state of the TlH molecule also indicate that the single-reference CISD calculation of the KRCI method could have an advantage over the conventional single-reference CI method based on the same REP. A Dirac-Fock-CI program, MOLFDIR [Comp. Phys. Comm. 81 (1994) 120], is modified to read the relativistic effective core potential (REP) integrals. The modified MOLFDIR, which can be used to perform multireference KRCI calculations, is applied to study the low-lying states of Tl2. For the ground state, the De of $\sim 0.22$ eV is obtained with the core 5d-valence correlation. The conventional configuration interaction method is not able to allow the excitations of 5d electrons due to a large reference set required to represent properly the dissociation of the ground state. Investigation of total energies of three low-lying ...