The convergence of spin–orbit configuration interaction calculations for TlH and (113)H

Abstract

To test the convergence of spin–orbit effects for molecules, the ground states 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 the Kramers’ restricted CI (KRCI) and the spin–orbit CI (SOCI) methods. The KRCI method includes the spin–orbit interactions in the generation of one-electron basis space through the use of the two-component molecular spinors obtained by the Kramers’ restricted Hartree–Fock (KRHF) method, whereas the SOCI adds the spin–orbit term only at the CI level. For systems with heavy atoms, orbital relaxations due to the spin–orbit interaction could become sizable, resulting in slow convergences for the SOCI method. Spin–orbit effects on bond lengths and energies using single- and multireference CI calculations at the SOCI level of theory are evaluated and compared with KRCI results for TlH and (113)H. The spin–orbit effects on energies converge easily for TlH but slowly for (113)H. Especially, bond lengths do not converge for the seventh-row (113)H in our calculations. The present results imply that large-scale multireference SOCI calculations are necessary for some molecules to recover orbital relaxation effects due to large spin–orbit interactions in the SOCI scheme. In those cases, the KRCI scheme based upon two-component spinors will have advantages over SOCI and other one-component orbital based methods.