Subject: SUMMARY: Hydrocarbon Forcefields

From: Craig Burkhart <[email-masked]>
Date: Tue, 05 Nov 1996 12:57:24 -0500

Dear Netters,

After much clacking of virtual tongues and over 25 responses from many parts
of cyberspace, I am finally delivering on the promised summary of responses
concerning liquid state simulations of alkanes/alkenes. Before proceding,
much thanks is in order to the following people--

On forcefields:

Bill Jorgensen (Yale), Ilja Siepmann (UMinn), David Maxwell (DuPont-Merck),
Erin Duffy (Yale) and Andreas Bick (MSI)

On methodology:

Mike Kotelyanskii (UDelaware), Ilja Siepmann, Eric Wallis (Phillips Petroleum)
and Carol Parrish (Columbia)

 ...And now on to the summary :)

My Queries:

1) Do you have experience with liquid density NpT MD simulations of
   branched hydrocarbons?

2) If so, can you point me in the direction of a forcefield which you
   have found appropriate for the calculation of liquid densities
   and delta-Hvap?

1) Concerning experience/methods:

Mike Kotelyanskii and Ilja Siepmann reminded me of an old rule of thumb
concerning cutoffs--that you should put them at a place where g(r) is
structureless; that is, where g(r) is close to unity. Good point. Most
of us learn this along the way and then promptly forget about it.

This brought up an important back-and-forth about the nature of how one
deals with long-ranged nonbonded interactions (van der Waals and
coulombic). Most of my simulations were underestimating the densities
of my alkanes by 5-7% or so. I was basically using nonbonded cutoffs
with smooth splines in about the 8-8.5 Angstrom range. This turns out
to be way too short, and if you look at the g(r) you realize that there
is significant liquid structure in this range. I redid a few of my
alkane simulations using 12-13 Angstrom cutoffs. The net result is that
I gained about 2% in density so that my errors were now in the 4% range
using Jorgensen's 1994 OPLS-AA.  For this I thank Mike Kotelyanskii for
being persistent enough in prodding me to look at the effect of
cutoffs.

We now turn our attention to the question of "tail corrections". Tail
corrections are done to handle the effect of truncating the nonbonded
potential. That is, it corrects the tail that exists past the cutoff.
For strictly van der Waals interactions, this correction is always
attractive (this may not be true with electrostatics).  The standard
method is discussed in detail in Allen & Tildesley's book "Computer
Simulation of Liquids".  Note however that this correction is most
applicable in hydrocarbon liquids for NVT ensembles. This is no problem
for software using Monte Carlo (MC) dynamics, which recalculates the
"tail term" only when a volume move, or some suitable interval, occurs.
My thanks to Eric Wallis in discussing his experience with MC tail
corrections.

It is however a problem in standard Newtonian (MD) dynamics using the
NpT (Gibbs) ensemble, where both molecules and system volume are
constantly changing. In this instance, using the Ewald summation may be
a more viable approach. A useful implementation of this method for both
coulombic and Lennard- Jones potentials is given by Karasawa & Goddard,
J. Phys. Chem. 93, 7320(1989). It is basically a generalization of
Donald Williams' "accelerated convergence" technique (see his series of
Acta Cryst.  articles in the late 60's-early 70's) which was based on
Ewald's seminal work. This newest rendition of the Ewald sum has its
advantages over systematically increasing your nonbonded cutoff.  It
computes an estimate of the full long-ranged interaction which is
dependent only on your convergence criteria. I've have used a 0.01
kcal/mole/atom cutoff with good success. The bottom line is that the
use of the Ewald method reduces my density errors to about 3% using the
1994 OPLS-AA.

2) Concerning forcefields:

There are 3 groups currently working on improving liquid alkane/alkene
forcefields, each with a slightly different emphasis. Bill Jorgensen's
group (which includes past members David Maxwell and Erin Duffy) has
published a great deal on their Optimized Potentials for Liquid
Simulations (OPLS) approach. Professor Jorgensen has informed me that a
new OPLS-AA forcefield will be published in an upcoming J. Amer. Chem.
Soc. November issue. He maintains that OPLS-AA(1996) will give
densities generally within 2%. With his help, I'm currently running a
test on a branched alkane liquid cell. My thanks to Bill Jorgensen
for helping me with the details of implementing OPLS-AA.

The second group involves Ilja Siepmann's recent collaborations.  He
has published work with B Smit, S Balasubramanian, C Mundy and ML Klein
on united-atom approaches to hydrocarbon liquid critical parameters
(critical density, temperature, ...) and solute partitioning.  His
approach may be appealing for polymer based problems as well. His most
recently published work [S Balasubramanian, ML Klein and JI Siepmann,
J. Phys. Chem. 1996, 100, 11960] use an updated forcefield. He informs
me that they, too, are putting the finishing touches on this
forcefield, which is due to be published next year; he has upcoming
articles submitted to Mol. Phys. and Fluid Phase Eq. which may be of
use to this audience.

The third group involves Andreas Bick and coworkers at MSI. They
are currently revamping the Biosym PCFF parameterization for liquids.
However, it is only available at this time to their consortium
members, and may be published sometime in the next year (we hope).

Summary:

The Jorgensen OPLS-AA forcefield, coupled with the appropriate
long-range corrections may be the most comprehensive method of
currently dealing with liquid hydrocarbons of small-to-intermediate
size. However, Siepmann's forcefield may be also useful depending
on the properties you wish to simulate, and may be more appropriate
for long-chain systems.

As always, I thank the members of the CCL for their help. Even if Jan
doesn't want to burn bandwidth on debating the CCL's survival, I
must add my $0.02 worth :). IMHO it is a unique GLOBAL resource and
that we should do what is necessary to guarantee its survival.
Time to put your money where your mouth is...

Sincerely yours,

Craig.

-- 
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