Re: SCRF for ions
Jaime,
>
> Hi all:
>
> I have been working with Gaussian 92 and Gamess using the SCRF method
> to study solvated anions. First, I found out that the two programs
> would give me different results for similar input decks!!!
> A closer look revealed that this
> difference is roughly equal to the molecular Born energy, which
> apparently is not accounted for in the Gaussian implementation of the
> self-consistent-reaction-field.
>
Your analysis is exactly correct, the monopole (or Born) term is not
included in the multipole expansion (which is truncated at the dipole in the
Onsager model).
In principle, one can take the G92 calculated spherical radius "a"
that
was used in the SCRF and use it to calculate the Born free energy from the
classical Born eqn G = -1/2 ( 1 - 1/dielectric) q^2/a, where q is the charge
on the molecule (all units atomic units).
Of course, in a charged molecule, the dipole is no longer origin
independent, and one needs to worry a bit about where the center of the
sphere is being placed -- center of mass? center of charge?
No matter what, the Born-Onsager model in a spherical cavity approximation
starts to get pretty risky about this point. Of course, it's fast and simple.
CJC
--
Christopher J. Cramer
University of Minnesota
Department of Chemistry
207 Pleasant St. SE
Minneapolis, MN 55455-0431
(612) 624-0859
cramer -AatT- maroon.tc.umn.edu
From mail Mon Oct 4 16:50:39 1993
Date: Mon, 4 Oct 1993 16:19:02 -0400
From: hyper!ostlund (Neil S. Ostlund)
Message-Id: <9310042019.AA19423 -AatT- hyper.hyper.com>
To: chemistry -AatT- ccl.net
Subject: conjugation in mol mech
John McKelvey has commented here about torsional angles in
biphenyls. HyperChem also gives reasonable values of torsional
angles in molecules such as this, but the issue is a very important
one that is fundamental to the success or failure of molecular
mechanics approaches and although I rarely contribute to
discussions here, I thought that I might briefly comment on this topic.
The standard molecular mechanics methods (MM2, Amber, CHARMm, etc)
assign parameters on the basis of the "atom type" of the relevant
atoms involved (4 atoms,in the case of a torsion) without consideration
of the bond type. This results in the same torsional constants for the
SINGLE BOND in biphenyl at for the AROMATIC RING BONDS of Benzene!!
Thus, all the standard methods mentioned above result in biphenyl
being planar which is unfortunate chemistry. The molecular
mechanics secret is to recognize more of the chemical environment of
a bond torsion than just the 4 "atom types". Alternatively, one might
expand the number of different atom types, but this has its own problems.
For example, in biphenyl it is important that one is trying to describe
the torsion of a "single" bond (bond-order=1) not an
"aromatic" benzene
bond (bond-order=1.5). Chemical ideas like this are not recognized by
the simple "atom type" methods, but are recognized by Dreiding, MM+,
and
others.
In HyperChem, the MM+ method does standard MM2 calculations when explicit
parameters are available in the parameter file associated with the relevant
atom types. Thus a "standard" calculation results in planar biphenyl
for the
reasons described above. However, the MM+ force field in HyperChem falls
back to a Dreiding-like scheme when explicit parameters for the torsion in
question are not available in the parameter file. It then uses information
about the "bond type" to derive default parameters. The result is
that you
get a better result when you don't have an explicit MM2 parameter and the
default scheme is used! This points out one of the major defficiencies of
standard molecular mechanics procedures like MM2, Amber, and CHARMm - they
don't recognize the "bond type" of a torsion but only the "atom
type" of
the 4 relevant atoms.
For those familiar with HyperChem, if you select the molecule and set all
the atoms types to unknown (**) to dismiss the standard MM2 parameters and
fall back to the default scheme, you will find reasonable geometries for
systems like biphenyl, butadiene, etc.
I don't want to sound as though I am promoting HyperChem here; I only
use it as an example of an important issue for molecular mechanics
calculations. Other programs also recognized this problem associated
with the simple "atom type" approach.
------------
Neil Ostlund
President, Hypercube Inc.
419 Phillip St, Waterloo, Ont, Canada N2L 3X2
(519)725-4040
internet: ostlund -AatT- hyper.com