Name
TRUES — Calculate the true solvation free energy.
Description
The keyword TRUES tells AMPAC to calculate the true solvation
free energy, which is the final “heat of formation” plus solvation free energy for the
geometry in solution minus the final “heat of formation” for the gas-phase geometry. (Notice
the use of the widespread convention by which “heat of formation” is used to denote the sum of
the electronic energy and nuclear repulsion with a particular choice for the zero of energy
and without including vibrational energy.) The TRUES keyword must be used in conjunction with
the HGAS=n.nn
keyword, where n.n is the gas-phase heat of formation. It is strongly recommended that the
gas-phase heat of formation be obtained with a geometry obtained using the same method in the
gas-phase and the liquid-phase optimization. This provides no problem when a rigid solvation
model is used (e.g., SM5.2R or SM5CR), in which case any reasonable gas-phase geometry may be
used in solution, though the models were parameterized using HF/MIDI! geometries. However,
when liquid-phase geometry optimization is performed, using SM5.2/X or SM5C/X, for example,
HGAS=n.nn should specify a gas-phase heat of formation calculated
with method X and using a geometry optimized in the gas-phase with method X.
