This keyword activates one of the AMSOL models contributed by D.G. Truhlar, C.J. Cramer, and G.C. Lynch. (See Chapter 15, AMSOL Model Module for a more complete description of the AMSOL methods and additional dedicated keywords.) By using both SM3 and PM3, the user requests a calculation in aqueous solution by the PM3-SM3 method. This is completely analogous to the SM2 method except that the underlying gas-phase Hamiltonian is the PM3 model and the solvation model parameters are based upon that approach rather than AM1. The most desirable feature of the PM3 model is that it appears to more accurately predict the properties of hydrogen bonds in terms of their linearity, distance, and energy than does AM1. A significant drawback in comparison to AM1 is that the PM3 nitrogen charges are usually much too positive and the SM3 model is thus reduced in its effectiveness when employed for amines, nitriles, and nitro compounds. In SM3 all accessible-surface-area parameters are independent of chemical environment, thus there is a unique value for all surface tension parameters for each atom type. PM3-SM3 is less accurate than AM1-SM2 (although more accurate than AM1-SM1) and may be used at the investigators discretion to compare AM1 and PM3 results or for cases where it is expected that PM3 treats the solute more accurately than AM1. Other than including the SM3 keyword, no modifications need be made to the standard AMPAC input file.
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