outputting forces in thermo_style custom

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1

Hi Lammps-users,

I am testing a many-body potential by finding forces between two atoms and
trying to output the results via thermo_style custom. Unfortunately, I
haven't been able to get thermo_style custom to work. Any advice would be
greatly appreciated!

1) Attempt w/compute
compute force all property/atom fx fy fz
thermo_style custom step etotal c_force
ERROR: Thermo compute does not compute scalar

2) Attempt w/compute
compute force all property/atom fx fy fz
thermo_style custom step etotal c_force[1]
ERROR: Thermo compute does not compute vector

3) Attempt w/ compute
compute force all property/atom fx fy fz
thermo_style custom step etotal c_force[1][1]
ERROR: Thermo compute does not compute array

I also tried the store/force fix:
4) Attempt w/ fix store/force
fix force go store/force
thermo_style custom step etotal f_force
ERROR: Thermo fix does not compute scalar

5) Attempt w/ fix store/force
fix force go store/force
thermo_style custom step etotal f_force[1]
ERROR: Thermo fix does not compute vector

6) Attempt w/ fix store/force
fix force go store/force
thermo_style custom step etotal f_force[1][1]
ERROR: Thermo fix does not compute array

What am I missing?

Thanks,
John

2

The doc page for thermo_style explains that the only computes
that can be used directly are "global" quantities. All of the ones
you are using are per-atom arrays or vectors. You should read
the howto section 4.15 about global, per-atom, local quantities.

The way to do what you want is define an equal-style variable
that has the force of the atom in question, which then becomes
a global scalar. Thus you can print the variable in the thermo output.

Steve