Hartree-Fock and second-order perturbation theory calculations were performed on complexes of a single alkali metal cation and a recently synthesized derivative of 18-crown-6 that locks the crown macrocycle into a D-3d-like configuration. Unlike 18-crown-6, which binds K+ with a 100-fold selectivity over Na+, the derivative prefers Na+ by a factor of 4:1. Calculations on a simple K+ <--> Na+ cation exchange reaction indicate a small shift in binding preference in favor of Na+, but the inherent uncertainty in the theoretical treatment makes an unequivocal conclusion difficult. The effects of two microsolvating waters attached to the cation/s18-crown-6 complex and up through four microsolvating waters bound to 18-crown-6 were considered. The levels of theory used in this study were similar to those previously found to yield binding preferences in qualitative agreement with Liquid-phase experimental data, ab the best of our knowledge, this work, which involved more than 800 basis functions for several complexes, represents the largest high-level ab initio study of crown ethers yet attempted.