Interactions between a Li+ cation and the bases NH3 and OH2 are studied by ab initio molecular orbital methods. Basis sets range in size from 6-31G* to 6-311 + G** and include also double-valence types containing two sets of polarization functions. Interactions energies computed with the singly polarized (SP) sets are too high, due in large part to overestimates of the dipole moments of the bases. Both of these errors are greatly reduced when d functions are added to first-now atoms (DP). The contributions of electron correlation to the interactions are included via Móller-Plesset theory to second and third orders. Whereas the SP basis sets lead incorrectly to attractive contributions of correlation, this component is positive with the DP sets which, moreover, involve substantially smaller basis set superposition errors at both the SCF and correlated levels. The second set of d functions is responsible also for correct behavior of both the second- and third-order components of correlation. Lithium cation and proton affinities calculated with the DP basis set are in excellent accord with values measured experimentally.