Low-lying states of Tl₂ calculated by the configuration interaction methods based upon relativistic effective core potentials and two-component spinors

We have modified a Dirac-Fock program package, MOLFDIR [L. Visscher et al., Comput. Phys. Commun. 81, 120 (1994)], to perform two-component molecular spinor calculations based upon the relativistic effective core potentials (REPs) incorporating effective spin-orbit operators. The modified MOLFDIR can be used to perform two-component REP calculations instead of four-component all-electron calculations at various levels of theory including the configuration interaction (CI) level. As a test case for the multireference CI methods, the low-lying states of Tl₂ are calculated with the two-component CI method and the more conventional spin-orbit CI method. Results indicate that the two-component method has some advantages in describing the ground state of Tl₂ while excited states are similarly described by both methods.