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

To test the convergence of spin-orbit effects for molecules, the ground sta tes of TlH and (113)H are calculated by configuration interaction(CI) calcu lations using relativistic effective core potentials with one-electron spin -orbit operators. The employed CI methods are the Kramers' restricted CI (K RCI) and the spin-orbit CI (SOCI) methods. The KRCI method includes the spi n-orbit interactions in the generation of one-electron basis space through the use of the two-component molecular spinors obtained by the Kramers' res tricted Hartree-Fock (KRHF) method, whereas the SOCI adds the spin-orbit te rm only at the CI level. For systems with heavy atoms, orbital relaxations due to the spin-orbit interaction could become sizable, resulting in slow c onvergences for the SOCI method. Spin-orbit effects on bond lengths and ene rgies using single- and multireference CI calculations at the SOCI level of theory are evaluated and compared with KRCI results for TlH and (113)H. Th e spin-orbit effects on energies converge easily for TlH but slowly for (11 3)H. Especially, bond lengths do not converge for the seventh-row (113)H in our calculations. The present results imply that large-scale multireferenc e SOCI calculations are necessary for some molecules to recover orbital rel axation effects due to large spin-orbit interactions in the SOCI scheme. In those cases, the KRCI scheme based upon two-component spinors will have ad vantages over SOCI and other one-component orbital based methods. (C) 2001 American Institute of Physics.