CCL:G: Relaxed PES in g03 using forcefield?
- From: Ödön Farkas <farkas=-=chem.elte.hu>
- Subject: CCL:G: Relaxed PES in g03 using forcefield?
- Date: Thu, 27 Sep 2007 10:28:39 +0200
I list a few facts about the ModRedundant option and optimization algorithms in
Gaussian, which might help you sorting out your problem:
1. I advise using redundant internal coordinates for PES scans.
2. There are default optimizers for different levels:
- MM optimizer is used for MM and ONIOM microiterations for the lowest level
MM calculation. ModRedundant option DOES NOT work but atoms in Cartesians can be
frozen via placing -1 after the atom symbol in the geometry part of the input.
At least 3 atoms should be frozen in Cartesians to avoid side effects. There is
no option to turn this algorithm on for other methods, Opt=NoMicro turns off
ONIOM microiterations and Opt=Large or.Opt=Small will lead to using redundant
internal coordinates even for MM calculations to allow the ModRedundant option.
Since using redundant internals for very large systems may be costly for MM, an
MM/MM ONIOM with microiterations and selecting the atoms which are involved in
ModRedundant for the "high" level allows doing PES scans for pure MM
calculations. In this case if both levels use the same MM setup the result will
be identical to using a simple MM calculation for the whole system.
3. The QM/MM ONIOM calculations use MM microiterations and
both, freezing atoms in the Cartesian space and using ModRedundant for the high
level can be used.
- Large molecular optimizer is default for semiempiricals, advised for large
molecules or ONIOM calculations in conjunction with NoMicro. Opt=Large turns
this option on for any calculation, Opt=(NoMicro, Large) is necessary to use it
for whole system optimization with ONIOM QM/MM jobs. Opt=ModRedundant works with
- Regular optimizer is usually the default and advised for QM calculations
and as the main optimizer for ONIOM jobs. Since only the higher level atoms are
treated with this optimizer in an ONIOM QM/MM job by default is is advised to
include all atoms belonging to ModRedundant lines into the high level
On Tue, 2007-09-25 at 10:23 -0700, Ben King king:-:chem.unr.edu wrote:
Sent to CCL by: Ben King [king^chem.unr.edu]
Summary: how can one use gaussian to perform a relaxed
energy scan with a force field?
We need to generate a 2-D potential energy surface for a
breaking coupled with a torsion. Gaussian's tools for
relaxed PES using the modredundant option to opt are
ideal, but Z-
matrix tools could also work. Here is the problem:
completely ignores any constraints when we use molecular
(amber, uff, etc.), using either Z-matrices or
This also happens in OMION methods using a MM fragment.
is correct, as we do these same calculations (albeit
only over limited portions of the PES) using
We must use a molecular mechanics force field: van der
interaction are important in our molecule and we need to
portion of the bond dissociation region that is poorly
single references wavefunctions (e.g., C-C bond distances
of 1.4 A to
6.0 A in 0.2 A increments). The molecule is large
(C62H38), so high-
level ab initio methods are prohibitively time consuming.
mechanics will give us the most realistic description of
molecular behavior, provided that we use a Morse potential
If anyone can suggest a procedure to generate a PES from a
field, I would be grateful. This is one of those
that should be easy but ends up being exceptionally
Also, if anyone knows why Gaussian behaves so oddly with
I would be delighted to know.
p.s. The version of Gaussian is: Gaussian 03:
p.p.s. We are limping through this by running thousands
individual constrained optimizations using Tinker, but it
Benjamin T. King, Associate Professor
Department of Chemistry/216
University of Nevada, Reno 89557
tel (775) 784-1736 fax (775) 784-6804
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