Quasirelativistic transition moment calculations using the multipartitioning perturbation theory: B0+(3Π)→X0+(1Σ+) transitions in IF and ICl

A simple method of ab initio quasirelativistic calculations on electronic transition moments in molecules is presented. The description of relativistic effects through effective core potentials is combined with the use of the second-order many-body multipartitioning perturbation theory for correlation treatment. Transition moment estimates are obtained by the finite-field technique. Pilot applications to the B0+(3Π)→X0+(1Σ+) transitions in IF and ICl are reported and the results are compared to experimental data. A new fit of the observed visible absorption spectra of ICl gives results in excellent agreement with our ab initio results.