Subject: Semiempirical vs. empirical force fields?

From: [email-masked] (Gert von Helden)
Date: Mon, 6 Feb 1995 18:39:18 +22305458 (PST)

Hi there,

I have a question regarding the quality of semiempirical vs. empirical methods.
We have been annealing small polyptides (9 amino acids, 150 atoms) using the amber
force field by repeatedly running MD at high temp., cooling it down and finally 
minimizing it. The standard amber ff has been used with a constant dielectric of 1.

We then take the lowest energy conformations and reoptimize them with PM3. 
The conformational changes are quite large with the PM3 geometries being somewhat
more open. Also, the relative energies change a lot.
In some situations conformation A is lower then B by 7 kcal/mol
using amber but B is lower that A by 5 kcal/mol using PM3.
We would like to compare the calculations to gas phase experiments. 
So which method is better and more reliable?

Thanks,   Gert

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+ Gert von Helden, Dept. of Chemistry, UCSB, Santa Barbara, CA 93106 +
+ Tel   : 805-893-2673, Fax   : 805-893-8703                         +
+ E-mail: [email-masked]                                     +
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