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 the SM2 and AM1 keywords, the user requests a calculation in aqueous solution by the AM1-SM2 method. While analogous to the SM1 method in its generality, the SM2 model recognizes the importance of classifying hydrogen atoms based on the heavy atom to which they are attached and derives that information from the bond order matrix. As a result, it is considerably more accurate and consistent than the SM1 approach. The output from such a calculation will include the final bond order matrix together with the contributions to surface-area-dependent terms from both heavy-atom surface tensions and attached hydrogen atoms (which do not block the solvent-accessible-surface-area of the heavy atoms to which they are attached). In SM2 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. Other than including the SM2 keyword, no modifications need be made to the standard AMPAC input file.
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