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Re: [lammps-users] LJ parames
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Re: [lammps-users] LJ parames

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Thu, 3 Aug 2017 08:38:02 -0400

On Wed, Aug 2, 2017 at 11:39 PM, 284237308@...1204... <284237308@...1204...> wrote:
Hello guys
    I'm all new here so I have a little stupid question for sigma and epsilon for LJ pair coefficients. In some papers, I find that they only give the whole epsilon and sigma between the molecules. However, when I write pair coeff command in lammps, it should be the epsilon and sigma between two atom types. For example, for water, it should be like,
TIP3P-E water
pair_style      lj/cut/coul/long 13.0 
bond_style      harmonic
angle_style     harmonic
kspace_style    pppm 1.0e-5
pair_coeff      1 1 0.0 0.0         #H-H
pair_coeff      2 2 0.102 3.188     #O-O
pair_coeff      1 2 0.0 0.0         #O-H
bond_coeff      1 450 0.9572        #H-O
angle_coeff     1 55 104.52         #H-O-H
    But the parameters for CH4 is written like epsilon=0.2937, sigma=3.73. I think it supposed to be three sets of epsilon and sigma for C-C, H-H and C-H.
    So how should I write lammps pair coeff commands correctly for CH4 with only one set of epsilon and sigma?

​in many molecular force field calculations, only epsilon and sigma are given for a specific atom type and not for all pairs of atom types.
the "mixed" parameters are usually inferred following a set of "mixing rules". LAMMPS does support this.​
you have to look up which mixing rules apply to the force field that you are using. typically, the geometric or arithmetic average is taken or a combination of them.

you can check what coefficients LAMMPS has generated for your system from applying the mixing rules by writing the full set of pair coefficients to a file with the write_coeff command:

as a beginner doing these steps and particularly on a rather simple systems as you are looking at, it is good practice to do this exercise and make certain that you have understood what LAMMPS will do automatically for you.


    Thanks a lot. I'm trying my best to learn everything I can. 


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Dr. Axel Kohlmeyer  akohlmey@...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.