Relativistic a b i n i t i o molecular structure calculations including configuration interaction with application to six states of TlH

A system for the inclusion of spin-orbit coupling along with moderate scale CI in calculations for molecules containing very heavy elements is demonstrated. In this effective potential procedure rigorous ab initio spin-orbit integrals are computed and added to the conventional integral set after the SCF and integral transformation steps of the calculation. This avoids the use of complex coefficients in the integral transformation and yet includes spin-orbit corrections on an equal footing with electron correlation. The diagonalization of the resulting complex CI plus SO matrix requires only about twice the time of a real CI diagonalization. Our present calculations on the two lowest 0+ and 1 states and the lowest 0− and 2 states of TlH indicate that this procedure allows adequate flexibility in the electronic coupling, resulting in bonding curves which are in good agreement with the experimentally established curves. The results also help to understand and to confirm previously conjectural interpretations of other spectral data.