Relativistic corrections to carbon atom energy levels and their relation to the singlet-triplet splitting in methylene

Abstract

The relativistic correction to the s 2 p 2-sp 3 excitation energy in the carbon atom is shown to be 120 cm-1 rather than 500 cm-1 as reported by Desclaux et al. This is too small to explain the 8 kJ mol-1 discrepancy between theory and experiment for the 1 A 1-3 B 1 energy separation in methylene. Two pitfalls that can arise when calculating relativistic corrections from approximate wavefunctions are discussed.