Subject: Re: CCL:G:rigid vs relaxed scan in Gaussian

From: [email-masked] (Per-Ola Norrby)
Date: Wed, 5 Jun 1996 12:11:17 +0200

Since I can't get through to Victor direct, I'll continue the
discussion here instead.

Victor M. Rosas Garcia wrote:

>Hi there,
>        I'm trying to calculate a bond stretching constant for molecular
>mechanics work.  I'm doing a Scan of the potential energy surface (well, curve
>in this case) for the stretching of the bond with the 6-31+G* basis set in
>Gaussian94.  The problem is that the keyword "Scan" in gaussian has two
>options: rigid and relaxed.  What is the recommendation for this kind of
>parametrization work?  Should the scan keep the molecule rigid or let it relax
>at each point of the stretch?  My boss says that it should be relaxed while I'm
>going for the rigid.  I think that letting the molecule relax at each point
>would make the constant too specific for the model compound and, thus, not of
>general applicability; not to mention cross-contamination by other stretches
>and torsions present in the molecule.
>Any comments are appreciated.
>
>Thanx
>
>Victor.

        Hi Victor,

        You have an interesting problem there.  The "standard" way to solve
it would be to use a relaxed scan (by the way, the recommended method in
g94 is to do this without SCAN, just using OPT and the "S" option in the
variable specification, either Z-matrix or IRC).  You would then have to
make sure that your MM method yields the same curve after parameterization.
This is a bit wasteful, and there IS a method to do it with rigid scans.
We have discussed the method for torsions (J.Comp.Chem. 16, 620-627, 1995).
The trick is to make sure you use the same EXACT geometries in both the ab
initio and molecular mechanics when you parameterize.  It's easiest with
torsions, since the energies aren't that high, and the couplings with other
parameters may not even be included in the force field.  If you are doing
stretches, and are working with a force field that includes a stretch-bend
term, you should probably sample the potential energy surface at several
points, not only changing the bond length but also angles.  If you are able
to include not only energies, but also energy derivatives in the
parameterization, it improves the results a lot.  For a beautiful example
of this and a discussion of the method, see Hagler et al., J.Comp.Chem. 15,
162-182, 1994.

        Note also that if you are staying at the relatively low level
HF/6-31+G*, you should use scaling.  A scale factor of 0.8929 is commonly
used in frequency calculation, Hagler et al. gives some other examples in
J.Am.Chem.Soc. 116, 2515-2525, 1994.  You could also take a look at
Halgrens recent parameterization in, I believe, the March or April number
of J.Comp.Chem. this year, he uses similar techniques but higher ab initio
levels for his parameterization.

        Best Regards,

        Per-Ola Norrby

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 *  Per-Ola Norrby
 *  The Royal Danish School of Pharmacy, Dept. of Med. Chem.
 *  Universitetsparken 2, DK 2100 Copenhagen, Denmark
 *  tel. +45-35376777-506, +45-35370850    fax +45-35372209
 *  Internet: [email-masked], http://compchem.dfh.dk/

---

From: "Victor M. Rosas Garcia" <[email-masked]>
Date: Wed, 5 Jun 1996 10:37:54 -0400

Hi people,

>Any comments are appreciated.

But my machine went down the day after (hard drive crash) and stayed like that
for a few days (fridays are bad days for maintenance).  So if some people tried
to reply directly to me, is must have bounced back.  Could those people kindly
resend me their replies? I'll post a summary of the answers.  So far I've got
the answers from Per-Ola Norrby, Artem Masunov and Andy Dustman (thanx).  Any
others?

TIA

Victor.

-- 
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Victor M. Rosas Garcia                   * "How can we contrive to be 
[email-masked]                *  at once astonished at the  
Virginia Tech doesn't necessarily share  *  world and yet at home in it?"
the opinions you just read.	         *  G. K. Chesterton
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