[gmx-users] how to scale epsilon parameters of Lennard-Jones term in .top file

Shuangyan Zhou zhoushy13 at lzu.edu.cn
Thu Dec 11 03:16:25 CET 2014

Hi all,
I am trying to run REST in GROMACS following the way proposed by tsuyoshi terakawa. In their method,they implemented REST by rescaling the force-field parameters without even changing the code. I have also searched in the mailing list and found that we can manipulate the scale parameter in ffnonbonded.itp and ffbonded.itp as well as the .rtp file for charges to construct the .top of highest "temperature" which corresponds to lambda=1.Taking the two states(the lowest "temperature" corresponds to lambda=0 regard as state A and highest temperature corresponds to lambda=1 regard as state B) into the same .top file, we can  we can directly use the Hamiltonian REM routine of GROMACS 4,which is based on the λ-dynamics and the linear combination potential scheme of the two-ends potentials(I do not know whether my understanding is right?).
I add the parameters of state B in to the topology A following the 5.7.4 "Topologies for free energy calculations" in GROMACS manual.The bond scaling parameters can be added in [bonds] with dA kA dB,kB,while the dA,kA,stand for the bond parameters of state A,and the dB,kB stand for the parameters of state B.Other scaling parameters such as [angles],[dihedrals] can be treat as the same way.My question is how to add the epsilon parameters of Lennard-Jones of state B into the .top file? The force field that I used is OPLS-AA force field,is it true add the epsilon value in the [pairs] part? I do not know in the [pairs] with the format of :ai    aj funct            c0            c1            c2            c3,what c0,c1,c2 and c3 stands for? 
Would you some body give me some suggestions to me? thanks very much

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