Electron structure calculations including CI for ten low lying states of Pb/sub 2/ and Sn/sub 2/. Partition function and dissociation energy of Sn/sub 2/

Relativistic calculations including configuration interaction are described for ten low lying states of Pb/sub 2/ and Sn/sub 2/. These calculations were accomplished by introducing spin-orbit interaction at the configuration interaction stage in calculations initiated in ..lambda..--S coupling. All the calculations were carried out with relativistic effective potentials which were averaged with respect to spin at the SCF stage and differenced for the spin-orbit operator. The selection of important configurations for the atoms and the molecules is discussed in terms of both ..lambda..--S and ..omega..--..omega.. coupling. Comparisons are made with the available experimental data. With the results of these calculations and recent experimental spectra, it is possible to revise the partition function for Sn/sub 2/. The partition function is required for the interpretation of mass spectral data on the dissociation energy. The optimum method for the calculation of the partition function where several electronic states are populated is discussed in general terms and then applied to the revision of the dissociation energy of Sn/sub 2/ with the result 2.04 +- 0.1 eV. An earlier revision for Pb/sub 2/ needs no further change.