[lammps-users] Erroneous non-bonded energies

Dear Steve,

First, thank you very much for your reply!


When 2 atoms are bonded the weighting coeff
is 0.0, you are turning off the vdwl and Coulomb
interaction between those 2 atoms, hence the
energy will change.

You are ABSOLUTELY right!
Moreover, this is just the thing I wrote in my first post. When we turn off vdwl and Coulomb interaction between those 2 atoms
the total vdwl and Coulomb energies of the system should alter by vdwl and Coulomb interaction between those 2 atoms.
Vdwl and Coulomb interaction between those 2 atoms are very easy to calculate. I did this in my first post.
(There vdwl interaction between those two atoms was zero since the distance between them was purposely chosen equal to sigma of Lennard-
Jones law (viz. 4 A) and Coulomb interaction was, of course, non-zero and equal to -83.015927)
And it was obvious that the total vdwl and Coulomb energies alter not by these calculated values but by some absolutely unexplainable values.
Namely, the difference in total vdwl energies was -5.19017 kcal/mol;
the difference in total Coulomb energies was -1858.34 kcal/mol.

I tried four solvers, viz.:

1) kspace_style none
   pair_style lj/cut/coul/cut 12.0

2) kspace_style ewald
   pair_style lj/cut/coul/long 12.0

3) kspace_style pppm
   pair_style lj/cut/coul/long 12.0

4) kspace_style ewald/n
   pair_style lj/coul long long 12.0

In all four cases the differences were the same.

Once more the conditions:
two atoms of one type;
charges are +1 and -1;
LJ coeffs: epsilon = 1 kcal/mol sigma = 4 A;
distance between them is 4 A.

What are interactions between them?
I claim that E_vdwl = 0 kcal/mol, E_coul = -83.015927 kcal/mol;
LAMMPS claims that E_vdwl = -5.19017 kcal/mol, E_coul = -1858.34 kcal/mol.
Who is right?

Best regards,


PS By the way, it's easy to calculate the non-bonded interactions
between two atoms. It's necessary to calculate the "non-bonded system"
(see data.AA in my first post) with "boundary f f f" and lj/cut/coul/cut.
The result is pretty good:

E_vdwl 0
E_coul -83.015928