*From*: "Dr./CPT Christopher J. Cramer" <cjcramer &$at$& crdec6.apgea.army.mil>*Subject*: AMSOL and MOPAC*Date*: Mon, 10 Feb 92 14:34:55 EST

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 ~ kpc23 &$at$& cas.org Internet kpc23 &$at$& cas Bitnet