AMBER Archive (2004)

Subject: AMBER: angle bending

From: Gustavo Pierdominici Sottile (gsottile_at_unq.edu.ar)
Date: Thu Aug 19 2004 - 07:31:21 CDT

Hi,
    I write this mail because I have a problem and can not resolve it.
The problem is the following:
I have parametrized a couple of streching and bending parameters, and have
never have any problem. But there is a bending that seems imposible to
parametrize.
The model consists of a benzene but with a nitrogen(B) replacing an
hydrogen.
 Moreover, there is a methyl group bound to that nitrogen.
The bending I want to characterize is that between the carbon of the
ring(A), the nitrogen(B)
and the carbon of th e methyl group(C).
To do this I make an energy plot with gaussian. The points where the energy
was calculated
have the same configurations, except for the angle A-B-C.
The following step was to calculate the non bonded energy to the same
configurations.
When substracting the non bonded energy to the ab initio one, the result is
not harmonic.
I will send you some values so as to add information about the problem:
The ab initio minimum is at 120 °, all the energy are in Kcal

angle ab energy non bonded difference
115 0.67 48.43 -47.76
120 0 46.99 -46.99
123 0.23 46.38 - 46.15

In other calculations I allways found a minimum value, but in this case when

the angle is below 120°, the difference energy decreases instead of
increasing.

Please if someone have had this problem or kwons what is happening, write me

Thanking in advance
Gustavo

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