trouble shooting a potential

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1

Dear Lammps users,

I am working on a pair potential and I am trying to troubleshoot a problem
where the system gains temperature very rapidly from a reasonable starting
configuration. I have checked the forces which seem reasonable. Is there a
way to do a sanity check if I have summed the virial correctly?

Thanks for the advice,
John

2

Dear Lammps users,

I am working on a pair potential and I am trying to troubleshoot a problem
where the system gains temperature very rapidly from a reasonable starting
configuration. I have checked the forces which seem reasonable. Is there a
way to do a sanity check if I have summed the virial correctly?

how does the virial play into this?

have you visualized the setup and the trajectory?

one common mistake is to have the wrong sign of the forces.

axel.

3

Hi Axel,

It is a modified Tersoff and it reduces to Tersoff at short range. I
checked the force output against the results from a pure Tersoff and it
matches. I was going through the code again trying to figure out what
could have gone wrong. I thought a bad virial might be a possibility and
was curious if there was a way to check.

If I do a small 128 atom graphene flake I get (can't even minimize):
Step Temp Press Volume TotEng
       1 1 nan 64000 nan

But for a minimized 5 atom system I get:
Step Temp Press Volume TotEng
      14 1 172.56 27000 -21.973804

The energy seems fine but the pressure looks goofy. Any ideas on what I
might be doing wrong?

Thanks again,
John

4

The energy seems fine but the pressure looks goofy. Any ideas on what I
might be doing wrong?

A bad virial means bad forces. If you're getting a Nan, then
one or more of your forces is also a Nan.

Steve