where each discretized surface area that belongs to atom j makes a contribution to the (strictly hydrostatic) compressive force. The resulting force field is added during a geometry optimization, until the restoring force of the molecule cancels the compression force field. It should be noted that the form of the potential causing F X-HCFF ext is not parabolic, making the evaluation of eq. 6 nontrivial. Both HCFF and X-HCFF are implemented in the Q-Chem program package. 2.3.2 Implicit Solvation Models: XP-PCM and GOSTSHYP