The effect of 3d shell back bonding on the binding of chlorine containing molecules

The description of the binding in molecules containing chlorine is studied with special emphasis on Cl 3d shell effects. Two difficult test examples were selected, ClF and CCl, where very poor results for the equilibrium bond distance have been reported earlier, even though large multireference CI calculations were performed. The earlier errors of nearly 0.10 a.u. in the bond distance are essentially removed by including the chlorine 3d shell in the CASSCF active space. The incorporation of the 3d shell in the active space allows for better backbonding from F 2p to Cl, and a better description of angular correlation. The best multireference contracted CI results for ClF are re =3.075 a.u. (expt. 3.077 a.u.), μe =0.885 D (0.888 D), and De =2.53 eV (2.67 eV). For CCl the ground state potential curve was calculated and essentially perfect agreement with experimental spectroscopic parameters was obtained. The C 2s orbital was not correlated in these calculations and the results for CCl may therefore be fortuitously good due to cancellation of errors. The poor performance of the multireference cluster correction for re in ClF and CCl is also discussed in the paper.