Is it possible to dump conservative forces only?

LAMMPS LAMMPS Beginners
1

Running the following input script with the thermostat commented out

units lj
dimension 3
atom_style atomic
pair_style lj/cut 2.5
boundary p p p

region box block -5 5 -5 5 -5 5
create_box 1 box
create_atoms 1 single 0 0 0
create_atoms 1 single 1 0 0
create_atoms 1 single 0 1 0

mass 1 1
pair_coeff 1 1 1.0 1.0

dump my_dump all custom 1000 "md.lammpstrj" id type x y z fx fy fz

velocity all create 1.0 4928459 mom yes rot yes dist gaussian
fix mynve all nve
# fix mylgv all langevin 1.0 1.0 0.1 1530917 zero yes
timestep 0.005
run 300000

yields a dump file that starts like this

ITEM: TIMESTEP
0
ITEM: NUMBER OF ATOMS
3
ITEM: BOX BOUNDS pp pp pp
-5.0000000000000000e+00 5.0000000000000000e+00
-5.0000000000000000e+00 5.0000000000000000e+00
-5.0000000000000000e+00 5.0000000000000000e+00
ITEM: ATOMS id type x y z fx fy fz
1 1 0 0 0 -24 -24 0
2 1 1 0 0 22.875 1.125 0
3 1 0 1 0 1.125 22.875 0

I verified by hand that the fx fy fz forces are the conservative forces as calculated by the lj/cut pair style. If I uncomment the thermostat line, the output changes to

ITEM: TIMESTEP
0
ITEM: NUMBER OF ATOMS
3
ITEM: BOX BOUNDS pp pp pp
-5.0000000000000000e+00 5.0000000000000000e+00
-5.0000000000000000e+00 5.0000000000000000e+00
-5.0000000000000000e+00 5.0000000000000000e+00
ITEM: ATOMS id type x y z fx fy fz
1 1 0 0 0 14.1429 -30.0936 60.4325
2 1 1 0 0 -14.5625 23.4622 -90.0978
3 1 0 1 0 0.419578 6.63142 29.6653

Clearly fx fy fz now reflect the effect of the thermostat.

For a force field machine learning project, I am explicitly interested in the conservative forces at every dumped timestep of an NVT simulation. Is there a way to dump the conservative forces only?

I already figured out that there are thermostats that do not seem to affect the forces, such as nvt, but naively replacing nve and langevin with fix 1 all nvt temp 300.0 300.0 $(100.0*dt) leads to lost atoms in this example.

2

No.

That means your starting configured is far from equilibrium.

You should be able to first bring the system to equilibrium with fix nve+langevin and then switch to fix nve later. If not, you have a more general problem with your simulation setup altogether.

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