The binding in ClF3

The binding energy of ClF3, with respect to ClF and F2, has been calculated using large multi-reference CI expansions and a variety of basis sets. The largest basis set included two sets of d functions on all centres and one set of f functions on chlorine. The contracted CI method was used. The best result obtained was +63 kJ mol-1 compared to the experimental value of +113 kJ mol-1. The SCF value is +21 kJ mol-1 and the CASSCF value -116·4 kJ mol-1. For the atomization energy a slightly different picture is seen, with the SCF value -96 kJ mol-1, the CASSCF value +84 kJ mol-1, the CI value +364 kJ mol-1 and the experimental value +532 kJ mol-1. Unlinked cluster contributions were found to be important. The bond distance in ClF was further optimized, which was also found to be a difficult problem. With f functions on both centres the multi-reference CI calculations resulted in an error of 0·03 Å, when CASSCF orbitals were used, and of 0·02 Å when SCF orbitals were used. The remaining errors for ClF3 and ClF are analysed. The interesting electronic structure of ClF3 is also discussed.