[gmx-users] CHARMM -> gromacs epsilon and sigma conversiion

[ABEL Stephane 175950]

Thank you !!! Jianhui
 
Stefane

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Message: 5
Date: Fri, 24 Sep 2010 14:18:25 -0600
From: Jianhui Tian <jianhuitian at gmail.com>
Subject: [gmx-users] CHARMM -> gromacs epsilon and sigma conversiion
To: gmx-users at gromacs.org
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        <AANLkTik8xPFHF66L78uxShh7TV6XfHREtc8HhbWJajMR at mail.gmail.com>
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eps(charmm)*4.184=epsilon(Gromacs)
0.15*4.184=0.6276

Rmin/2(Charmm)*2/(2^1/6)=sigma(Gromacs)
2.27*2/(2^(1/6))=0.404468(nm)

Cheers,
Jianhui

Date: Fri, 24 Sep 2010 15:55:44 +0200
From: "ABEL Stephane 175950" <Stephane.ABEL at cea.fr>
Subject: [gmx-users] CHARMM -> gromacs epsilon and sigma conversiion
       factors
To: <gmx-users at gromacs.org>
Message-ID:
       <F654B3EE96986E4B8DC6EF0919C88DA301038BAD at LODERI.intra.cea.fr>
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Hi all,

I would like to add new  atom types in the charmm27.ff for futures
simulations of glyolipids in water with gmx 4.5.1. I have finished to
convert the bonded in GROMACS format. However, I am little puzzled with the
vdW paramaters. I would like to know how to convert them (i.e. what are the
conversion factors used to convert the CHARMM sigma and epsilon parameters
to gromacs sigma and epsilon values ?).

For example for the chloride atom CLA

CHARMM 27

; atom  ignored    epsilon      Rmin/2   ignored   eps,1-4       Rmin/2,1-4
CLA      0.0       -0.150     2.27      ! chloride

GROMACS ffcharmmnb.itp

;name at.num mass charge ptype sigma epsilon
CLA 17 35.450000 -1.00 A 0.404468018036 0.6276

Thanks in advance for your help

Stefane
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