LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Mailing List Archives
Re: [lammps-users] LJ parames
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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.​ http://lammps.sandia.gov/doc/pair_modify.html
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: http://lammps.sandia.gov/doc/write_coeff.html

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.

​axel.​


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


赵骁
西安交通大学
动力工程多相流国家重点实验室

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
lammps-users mailing list
lammps-users@...6297....sourceforge.net
https://lists.sourceforge.net/lists/listinfo/lammps-users




--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.