Subject: Re: CCL:MM calc'n of stereoisomers

From: "Dr. Tom Pierce" <[email-masked]>
Date: Wed, 20 Apr 1994 16:23:27 +22305458 (EDT)

Assuming that you are still asking the question..

Sure - The comparisons will be as accurate as the  forcefield .
Note that in a multiple-minima problem  you have to check 
your minimia. 

(Step 1 finding an energy for
a isomer - this needs to be checked to see if the minimizer is
locked in a local minimum, ie use different starting points..
step 2 - invert the chiral center and minimize.. Note that you also
will need somewhat different starting points after the inversion, since
your minimizer will likely get stuck in some local minimum. 

The reason that you might need to try different starting points
is that usually a minimizer will at the beginning of a minimization,
chose a large step change for minimizing the energy. This step change
is very dependent on the initial derivatives and can push a molecule
into a different local minimum.

Step Three - Is there a difference? Is it within the error of the 
forcefield? If it is within the error of the forcefield then
there is no practical difference..

-- 
Sincerely, Thomas Pierce - [email-masked]  -  Computational Chemist
"These opinions are those of the writer and not the Rohm and Haas Company."

---------------------------------------------------------------

---

From: "David C. Doherty" <[email-masked]>
Date: Wed, 20 Apr 1994 16:24:14 -0600

>What I was
>concerned with is the comparison of diasteriomers with mechanics.  Can the
>strain energies of diasteriomers (sterioisomers that are not mirror images)
>be compared with any validity?

Most certainly. For instance, there have been many successful studies using
MM to look at relative enrgetics of syndiotactic vs. isotactic polymers.

For instance, 

@article{Doherty_and_Hopfinger_(1989),
        author = "David C. Doherty and Anton J. Hopfinger",
        title = "Molecular Modeling of Polymers. 6. Intramolecular
Conformational Analyses and Molecular Dynamics of Syndiotactic
Polystyrene",
        journal = "Macromolecules",
        year = "1989",
        volume = "22",
        pages = "2472"

looks at the energetics of syndiotactic polystyrene, but also contains
references
to its diastereomer, isotactic polystyrene.

---
David C. Doherty
Minnesota Supercomputer Center, Inc.
[email-masked]