On the convergence of spin-orbit configuration interaction calculations for TlH and (113)H

Abstract

The ground state of TlH and (113)H are calculated by configuration interaction(CI) calculations using relativistic effective core potentials with one-electron spin-orbit operators. The employed CI methods are Kramers`` restricted CI (KRCI) and spin-orbit CI (SOCI). The KRCI method can include the spin-orbit effects even from the formation of the orbital space through the use of the two-component molecular spinors obtained by the Kramers`` restricted Hartree-Fock (KRHF) method, whereas the conventional scheme, SOCI, for the spin-orbit effects adds spin-orbit term only at the CI level. For systems with heavy atoms, orbital relaxation effects due to the spin-orbit interaction are expected to become sizable, which could make the SOCI method suffer from slow convergence. Spin-orbit effects on bond lengths and energies using single- and multi-reference CI calculations at the SOCI level of theory are evaluated and compared with KRCI results. The spin-orbit effect on energies was converged for both TlH and (113)H, however on bond lengths was not converged for the seventh-row (113)H in our calculations. It required large scale MRSD-SOCI calculation to recover large orbital relaxation effects due to spin-orbit interaction at SOCI scheme.