Re: WSN: Constant pressure (fwd)

To: Water Science Network <water@gibbs.oit.unc.edu>
Subject: Re: WSN: Constant pressure (fwd)
From: Water Science Network Administrator <wsn-adm@mmlds1.pha.unc.edu>
Date: Mon, 18 Apr 1994 21:01:46 -0400 (EDT)

Sender: Dan Severance <dan@omega.chem.yale.edu>
Subject: Re: WSN: Constant pressure

> Anyone out there have experience with moderate-length (> 20 ps) MD
> simulations of TIP3P water at constant pressure?  When I run for 40 ps
 ....
> averaged over the last 20 ps are very much flattened out.  When I run
> for ~25 ps (~5 ps heating & equil, 20 ps measurement), the RDF's over
> the last 20 ps look reasonable.
> 
> Can anyone suggest a way around this?  (I've already decreased the
> compressibility parameter (in CHARMm) from 4.63e-5 to 1.0e-6, with no
                                ^^^^^^
> apparent effect.)  Or is 20 ps somehow an intrinsic limit to the
> stability of this particular simulation?
> 
    Hi Jay,
       What parameters are being used?  Last I'd heard, TIP3P in CHARMm
    is not actually TIP3P, but a modified version of it.  They've added
    Lennard-Jones parameters on the hydrogens.  "Real" TIP3P has no
    Lennard-Jones parameters on the hydrogens and has rigid bond
    lengths and angles (0.9572 Ang. bond lengths and 104.52 degree bond
    angle).
       I've never run the CHARMM version of what they call TIP3P to
    find out what it's pure liquid characteristics are, but it would be
    interesting to know.
       Other than that, I can't help you much, I just wanted to point
    out that there is no guarantee that their modified model will give
    the results that Prof. Jorgensen has published.
       By the way, I'm a post-doc in Prof. Jorgensen's lab :-)

          Good luck,
             Dan Severance
             dan@omega.chem.yale.edu

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