Lammps energy matching with two atoms

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1

Dear all,
               I am a naive user of lammps and want to calculate
coulomb energy between two atoms. The charges on each atom is +1.0. I
use the following input file for lammps,

units real
atom_style full
pair_style lj/cut/coul/long 10.0
kspace_style pppm 1.0e-4
read_data twoatoms_check_autopsf.data
timestep 1.0
thermo_style custom evdwl ecoul epair emol elong etail
thermo 0
run 1

The coulomb energy as in the output file as follows:

E_vdwl E_coul E_pair E_mol E_long E_tail
           0 106.01143 -94.573617 0 -200.58504 0

The distance between two atoms is 9.29. I think the coulomb energy
value is too high. I want to ask whether the procedure that i followed
is correct. Why you expect such a high value coulomb energy for two
atoms.?

Thanks for a reply in advance,
Mohan Maruthi

2

Since you are using a kspace_style, the total coulombic energy is ecoul + elong.

Ray

3

Since you are using a kspace_style, the total coulombic energy is ecoul +
elong.

Dear Mohan

   Forgive me if you already know this, but

This energy (ecoul+elong) represents the energy of these two
point-charges interacting directly with each other, as well as the ALL
of their periodic images. (This is an infinite sum calculated
approximately using kspace_style pppm.)

    If for some reason, you want to only want to calculate directly
the coulombic energy between two point charges (without periodic
boundary conditions), then:

---- try replacing: ----

pair_style lj/cut/coul/long 10.0
kspace_style pppm 1.0e-4

---- with: ----

pair_style lj/cut/coul/cut 10.0
#kspace_style pppm 1.0e-4
boundary f f f

Cheers
Andrew

For more information, see:

  Also read about "kspace_style msm"