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