This keyword activates the SM2.1 AMSOL solvation model and the force trapezoid radial quadrature scheme contributed by C. J. Cramer, G. D. Hawkins, G. C. Lynch, D. G. Truhlar, and D. A. Liotard. (See Chapter 15, AMSOL Model Module for a more complete description of the AMSOL methods and additional dedicated keywords. All other SMx models use the force rectangle scheme for radial quadratures, resulting in slightly less accurate evaluations of these quantities and slower speed.) The keyword SM2.1 together with the AM1 keyword requests a calculation in aqueous solution by the AM1-SM2.1 method. This is completely analogous to the SM2 method except that it was parameterized using essentially converged radial quadratures in the dielectric screening algorithm, as calculated by the force trapezoid rule, whereas in SM2 the parameters were obtained using unconverged quadratures carried out by the force rectangle rule. The force trapezoid rule is more efficient and more stable than the force rectangle rule, and it is recommended for that reason. Except for small changes in the parameters and the use of a different radial quadrature scheme, the SM2.1 model is the same as SM2, and the results are generally very similar. The force trapezoid quadrature scheme, however, allows use of the TEXPN, TONE, and EXTM options, which are unavailable in SM1, SM1a, SM2, and SM3. Greater computational efficiency of the force trapezoid rule and/or the use of one or more of the TEXPN, TONE, and EXTM keywords are the main reasons why the SM2.1 option would be chosen over one of the previous models.
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