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

High-level ab initio calculations are performed to determine the lowest sin glet-triplet (S-T) gap in CCl2, CBr2 and CI2. For CCI2 all-electron basis s ets are used. For CBr2 all-electron and effective core potential (ECP) basi s 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 tha t 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. S ome comments are made regarding the disagreement with recent photodetachmen t studies. (C) 2000 Elsevier Science B.V. All rights reserved.