Water water everywhere but not a drop to drink

From: d3f012 at.at pellucidar.pnl.gov
Subject: Water water everywhere but not a drop to drink
Date: Tue, 8 Jun 93 09:17:46 -0700
 I am curious about the way water is treated in the Discover
 code from Biosym.  The Discover parameters below are
 from the Consistent Valence Force Field (CVFF) used by
 Discover.
 Biosym uses what appears to be some variant of SPC water.  I
 give the relevant parameters below for comparison.  For
 reference on SPC, I use the article by Jorgensen et. al.
 "Comparison of simple potential functions for simulating
 liquid water"  J. Chem. Phys. 79 (1983) p. 926.
                   SPC                Discover (CVFF)
 ------------------------------------------------------
 rigid water       Yes                  No
 r(OH) Ang.        1.0                 0.96
 angle (HOH) deg.  109.47              104.5
 Charge(O)         -0.82               -0.82
 Charge(H)          0.41                0.41
 Ax10^-3,
 kcal A^12/mol     629.7               629.358
 C
 kcal A^6/mol      625.5               625.5
 The HOH angle of 109.47 for SPC water was required to
 fit the O-O radial distribution for liquid water,
 in particular the hump at ~4.5 angstroms (see above
 reference).  The bond angle of 104.5, used in CVFF,
 is more in line with the gas-phase value.
 As well, SPC uses rigid water, while CVFF is a flexible
 water model with Morse potential for O-H and quadratic
 term for the HOH bond angle.
 My questions:
      Have there been any tests, published or nonpublished,
      to verify the modifications used by Biosym.
      Also, Discover, apparently does not implement SHAKE
      so that rigid water models could be used.  Is the
      reason for this scientific or pragmatic?
 **************************************************************************
 Mark A. Thompson
 Sr. Research Scientist              email:  d3f012 at.at pnlg.pnl.gov
 Molecular Science Research Center   FAX  :  509-375-6631
 Pacific Northwest Laboratory        voice:  509-375-6734
 PO Box 999, Mail Stop K1-90
 Richland, WA.  99352
 Argus available via anonymous ftp from pnlg.pnl.gov (130.20.64.11) (in the
 argus directory).  Download the README file first.
 Disclaimer:  The views expressed in this message are solely my own and
              do not represent Battelle Memorial Institute, Pacific
              Northwest Laboratory, or any of its clients.
 **************************************************************************