AMSOL and MOPAC

Netters,
    The message attached below describes a chimeric MOPAC version modified by
 inclusion of the subroutines from AMSOL which calculate aqueous free energies
 of solvation. AMSOL is Cramer and Truhlar's modification of AMPAC 2.1.
    I must point out that this simple transfer of subroutines is doomed
 because of the way in which the two programs calculate derivatives with
 respect to nuclear motion, i.e. geometry optimization. AMSOL requires use of
 the keyword 'DERINU'. It is unfortunate that this keyword 'appears' to be a
 request for derivatives calculated numerically. In fact, the default
 optimization techniques for both AMPAC and MOPAC calculate derivatives
 numerically. Moreover, the gradients are calculated as the sum of all
 pairwise contributions from every unique two-atom pair where one of the atoms
 HAS BEEN MOVED. Without getting bogged down in the details, this is a pleasantly
 fast procedure because the energy calculations involve rather small matrices --
 at most 8 by 8. However, the theory underpinning AMSOL does not allow this
 decomposition to be made -- ALL pairwise interactions are affected by the
 movement of a single atom, specifically the Coulomb radii used in the
 calculation of G(ENP) are changed purely as a result of the new geometry.
 Moreover, the movement of any one atom may cause a significant change in both
 the density and Fock matrices as a result of charge redistribution, and the
 above approach does not permit the Coulomb radii to relax accordingly. It is
 thus deficient in two significant respects. To arrive at true stationary
 points, gradients must be calculated using the full Fock and density matrices.
 This is what 'DERINU' accomplishes. From my perusal of MOPAC 6.0 such an option
 is not incorporated into that code (indeed, the AMPAC code makes it clear that
 this is regarded as somewhat archaic, but with solvation effects turned on it
 can not be avoided).
    Thus, structures optimized by the modified MOPAC are NOT true stationary
 points, although they may be quite close depending on the molecule in
 question. In general, however, molecules which are either highly polarizable
 or incorporate atoms with large partial charges will not be treated well by
 the pairwise-summation approach.
    We are not advocating AMPAC over MOPAC (or vice versa) in any way. It
 would not be terribly difficult to modify the MOPAC subroutine DCART.f to
 permit the equivalent gradient calculations to be performed. However, the
 latest version of MOPAC does not come so equipped, and that is one of the
 chief reasons we chose to create AMSOL as opposed to "MOSOL".
 Chris Cramer
 Date: Mon, 10 Feb 92 17:05:33 GMT
 From: James P. Schmidt <jims &$at$& duce.medicine.rochester.edu>
 To: CHEMISTRY &$at$& ccl.net
 Subject: amsol
 Date: Mon, 10 Feb 92 16:58:11 EST
 From: kpc23%CAS.BITNET &$at$& OHSTVMA.ACS.OHIO-STATE.EDU (Kevin P. Cross Ext.
 3192 Room 2209B)
 Subject: Useful Numeric Property Data
 To: CHEMISTRY &$at$& ccl.net.bitnet
 Status: R
 Dear netters:
      Chemical Abstracts Service is interested in
 the types of numeric data you would like to see
 associated with structures and how you would use
 such data.  This could be data for molecular modeling,
 computational chemistry, etc.  In particular, we
 are interested in useful types of data that could
 be calculated for large numbers of structures
 that users might find beneficial when searching
 in conjuction with structures.
 We welcome any and all suggestions!
                         Sincerely,
 ~
                         Kevin P. Cross, Ph.D.
                         Research
                         Chemical Abstracts Service
                         2540 Olentangy River Road
                         P.O. Box 3012
                         Columbus, OH 43210
 ~
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