CALCULATING ELECTROSTATIC FORCES FROM GRID-CALCULATED POTENTIALS

The accurate calculation of forces from finite difference potentials is very important, especially in the area of Brownian dynamics simulations. Test charge methods are typically used to calculate these forces. In these methods, the potential is calculated with one group of charges present, then the force on a second set of charges is calculated as the negative of the gradient of the potential times the charge. The test charge methods for calculating forces between solute molecules have been compared with more accurate methods and then regions of validity of the test charge methods explored. The test charge methods neglect certain reaction field effects. It is found for the simple charged systems studied that beyond a center‐to‐center separation of about twice the sum of the molecular radii the test charge approximations can be quite good. For polar molecules with no net charges, however, the corrections can be significant to even longer ranges. Copyright © 1990 John Wiley & Sons, Inc.