From d3f012 ^at^ pellucidar.pnl.gov  Tue Jun  8 02:17:46 1993
Date: Tue, 8 Jun 93 09:17:46 -0700
From: d3f012- at -pellucidar.pnl.gov
Subject: Water water everywhere but not a drop to drink
To: chemistry |-at-| ccl.net
Message-Id: <9306081617.AA28311 _-at-_)pellucidar.pnl.gov>





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^ 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.
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