TABLE II.

Comparison of gradients computed via Eq. (A1) to those computed with finite differences using a displacement of 10⁻⁴ Å. The grid spacing is in Å. MAE is the mean absolute error, and max is the maximum absolute error (both in kcal/mol-Å) for the x, y, and z components of the gradient for 20 randomly selected atoms. Δμ is the excess chemical potential in kcal/mol. The final four columns give the excess number of water molecules and ions. The system is one unit cell of the sarcin–ricin system PDB ID 480d, with 108 nucleotides and a solute charge of −104e. The solvents are 0.02M MgCl₂ and 0.14M KCl in water.

Grid spacing	MAE	Max	Δμ	H2O	Mg2+	K+	Cl−
0.75	0.0053	0.0332	82.7	−1049.6	19.89	56.79	−7.43
0.50	0.0026	0.0128	54.2	−1047.6	19.89	56.79	−7.43
0.25	0.0006	0.0040	43.9	−1047.3	19.89	56.79	−7.43
0.15	0.0004	0.0018	43.6	−1047.2	19.89	56.79	−7.43