The lowest singlet–triplet gap in CCl2, CBr2 and CI2

High-level ab initio calculations are performed to determine the lowest singlet–triplet (S–T) gap in CCl2, CBr2 and CI2. For CCl2 all-electron basis sets are used. For CBr2 all-electron and effective core potential (ECP) basis sets are used, whilst for CI2 only ECP basis sets are used. Methods up to CCSD(T) are employed, as well as the B3LYP method. The conclusions are that for CCl2 the S–T gap is 20±1 kcal mol−1 and that the gaps for CBr2 and CI2 are 17±2 kcal mol−1 and 9±2 kcal mol−1, respectively. Some comments are made regarding the disagreement with recent photodetachment studies.