Effect of short- and long-range coulomb energy on the pressure of the system

Dear all,
I am modeling a system in which long range coulomb interactions (short- and long-range) are dominant. I found that the coulomb energy of the system is relatively insensitive the cut-off selected, which meets my expectations. The figure on the left shows that as the cut-off increases the short-range coulomb energy increases while the long-range decreases. The sum of both energies balances out leading to a coulomb energy relatively insensitive to the cut-off.

However, the pressure in the system is extremely sensitive to the cut-off and seems to only follow the short-range coulomb energy without taking into account the long-range coulomb energy. This seems odd to me.

The energy and pressure values plotted are those at the end of a minimization of the atomic system. The interatomic potential is defined as:

kspace_style ewald 1.e-003
pair_style lj/class2/coul/long 10.5 10.5

The cut-off in question is the second parameter of the pair_style command (it corresponds to the cut-off of the coulomb interactions).

I wonder if there a way to include the long-range coulomb interactions in the calculation of the pressure in the system, or if my expectations are completely off the mark. I am using a version of lammps update on Nov. 5, 2015.

Thanks a lot,

image.png

Attached: msword document including figures.

Effect of short- and long-range coulomb energy on thepressure of the system.docx (66 KB)

Dear all,
I am modeling a system in which long range coulomb interactions (short-
and long-range) are dominant. I found that the coulomb energy of the system
is relatively insensitive the cut-off selected, which meets my
expectations. The figure on the left shows that as the cut-off increases
the short-range coulomb energy increases while the long-range decreases.
The sum of both energies balances out leading to a coulomb energy
relatively insensitive to the cut-off.

However, the pressure in the system is extremely sensitive to the cut-off
and seems to only follow the short-range coulomb energy without taking into
account the long-range coulomb energy. This seems odd to me.

The energy and pressure values plotted are those at the end of a
minimization of the atomic system. The interatomic potential is defined as:

kspace_style ewald 1.e-003
pair_style lj/class2/coul/long 10.5 10.5

The cut-off in question is the second parameter of the pair_style command
(it corresponds to the cut-off of the coulomb interactions).

I wonder if there a way to include the long-range coulomb interactions in
the calculation of the pressure in the system, or if my expectations are
completely off the mark. I am using a version of lammps update on Nov. 5,
2015.

​pressure *is* included in ewald summation. ​the problem is your very large
ewald sum convergence parameter​. pressure is very sensitive to this, much
more than total energy. at 1.0e-3 you are not very well converged. try with
1.0e-6 or smaller.

axel.