Table 2.

Effect of Na⁺ and K⁺ basis sets on computed gas phase Gibbs free energies (kcal/mol).^†

Basis Set	ΔGNa+(H2O)3→Na+(H2O)2+(H2O)(g)		ΔGK+(H2O)3→K+(H2O)2+(H2O)(g)
	Estimated	% Error	Estimated	% Error
Expt.a	9.5		6.3
6-31G(d)	9.6	1.1	5.8	7.9
6-31+G(d)	9.6	1.1	5.6	11.1
6-31+G(2d)	-	-	5.4	14.3
6-311+G(2d)	-	-	6.4	1.6

^†Our goal here is to find the smallest basis set that estimates the experimental free energy difference between two randomly selected ion-water complexes within a 2% error. To accomplish this, we start with the 6-31G(d) basis set for each ion and gradually increase its size. We keep the basis set for the water molecule fixed at 6-31++G(d,p) in accord with the observation³⁷ that it yields the energetic and structural properties of dipole-dipole interactions, H₂O-H₂O and H₂O-HF, in excellent agreement with experiment.

^aReference ³⁹