# Re: [AMBER] torsion parameters in parm99.dat

From: Dmitry Nilov <nilovdm.gmail.com>
Date: Wed, 15 Dec 2010 10:21:24 +0300

Hello!
Load Amber8 manual from http://ambermd.org, and in "Parameter
development" section you will find the explanation.

On Wed, Dec 15, 2010 at 9:42 AM, xqkong <xqkong.mail.shcnc.ac.cn> wrote:
> Dear amber users,
>    As a novice of amber, i have some questions about the torsion parameters in parm99.dat. I noticed that sometimes there are  more than one line of torsion parameters corresponding to the given atom types.Such as
> CT-CT-CT-CT   1    0.18          0.0            -3.
> CT-CT-CT-CT   1    0.25        180.0            -2.
> CT-CT-CT-CT   1    0.20        180.0             1
> Therefore, I have the following questions,
>   First of all ,why do we need three  kinds of parameters to describe a given torsion angle ? what does the chemical meaning of this configuration ?I noticed  that the difference in the periodicity range from 3 to 1.But we
> know that when we rotate  a bond  consists of  four sp3 carbons from 0 to 360 degree, the number of  the maximum energy should be 3. so the
> periodicity should be 3 rather than 2 or 1.
>  Secondly,  How does the program such as sander to use these
> parameters in the simulation? In amber force field , the torsion energy was
> defined as follow,
> ET=sum{Vn/(2*idiv)*(1+cos(|pn|*phi-phase)}
> Taking the CT-CT-CT-CT as an example, should the total torsional
> energy for this dihedral be
> E(torsion for CT-CT-CT-CT  )
> = ( pk1*(1 + Cos[pn1*phi - phase1]) + pk2*(1 + Cos[pn2*phi - phase2]) + pk3*(1 + Cos[pn3*phi - phase3]) ).where pk1 = 0.20; pk2 = 0.25; pk3 = 0.18; phase1 = Pi; phase2 = Pi; phase3 = 0;  pn1 = 1; pn2 = 2; pn3 = 3 ?
>    Any suggestions will be greatly appreciated!
>
> Best regards,
> Xiangqian Kong
> Drug Design and Discovery Center,
> Shanghai Institute of Materia Medica,
> The Chinese Academy of Sciences,
> Shanghai 201203,P. R. China
> Email: xqkong.mail.shcnc.ac.cn
>
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>

```--
Dmitry Nilov,
Lomonosov Moscow State University
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Received on Tue Dec 14 2010 - 23:30:04 PST
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