The weakly avoided crossing between the two lowest (1)Sigma (+) electronic states of a series of alkali halide molecules has been studied by means of the recently reported multistate complete active space second-order perturbation theory, MS-CASPT2, method. For a large enough basis set and a complete active space self-consistent field that includes part of the radial and angular correlation of the outermost halide electrons, the calculated crossing distance is in very good agreement with that predicted from the Rittner empirical potential. The study of the relevant parameters corresponding to the crossing region on these molecules has been extended to include the effect of a uniform electric field and a generalization of the empirical Rittner formula that includes the electric field effects is presented. The predictions made by the MS-CASPT2 method are also in agreement with those derived from the generalized Rittner potential. Finally, the possible implications of the present work on electron transfer processes at metal electrodes are discussed.