# [lammps-users] FW: system can't get real energy minimum during minimization

Hi,all.

I encounter a problem that I can’t figure out what’s wrong with it. I think the box didn’t reach real energy minimum when it end running.

I want to get surface stress of Au (001) surface. My idea is modeling a big piece of block with 2 free (001) surface, so I set xy periodic boundary conditions and z is free.

Then I ask Lammps give me the pressure pxx pyy pzz. This pressure is generated by surface stress. I mean, if all the three directions xyz is periodic boundary conditions,

no pressure would be there when I use equilibrium lattice parameter to create atoms.

Here is one log file:

Dear Li,
Try to run a series of Minimizations and MD (in NVE). Each time you run MD after minimization, your stress will result in some kinetic energy of the atoms due to forces. Since total energy is constant, potential energy will get lower. After certain steps, you set the velocities to zero and minimize the system again. When you do that couple of times, you will see your total energy decreasing with each iteration as well as decrease in residual pressure.

Regards,
Vikas

There is no KE effect for minimization, since only the potential
energy is being minimized. However, Vikas's suggestion to run
the minimization multiple times is a good one. You might also
try other linesearch or min_styles to see if you get closer to zero pressure.

However, I don't think there is any requirement that the Pzz go to zero
when you have a free surface in z (non-periodic box). It looks like
the forces are becoming very small, and pressure is a highly sensitive
quantity in a solid, so I think you have already converged to a quite
good solution with a quite small Pzz.

Steve

Steve

Dear Steve and Vikas,

How to run minimization multiple times?
I want the system relax more, so I set energy and force tolerance to be zero, set the max iterations a big number, but it stops at 63 steps anyway.
So, exactly how to do that?

Best regards,
Weina

The minimize command is like the run comand. It can
be listed multiple times in your input script, and settings
for the minimizer, or the min_style can be changed in between.
Note that if you reset the min_style, all previous settings
for it are wiped out, and reset to defaults.

Steve

2009/10/9 Li, Weina <[email protected]>: