I think I have a misunderstanding of fix box/relax and I’m hoping for some clarification. If I take a crystal structure at the minimum energy structure from a DFT calculation and apply fix box/relax iso 0.0 and perform a series of minimizations, the forces slowly decrease as the box expands until the final minimization converges, as I expected. However, if I uncomment the minimization command in the input script below that occurs before the fix box/relax is called, the pressure value printed in the thermo data after the first minimization is very large. At zero temperature, I expected that for a minimized structure, the pressure should be near zero, since only the virial terms play a role, and the forces are near zero. Once the fix box/relax minimizations start, the forces start to grow as the box increases in size. What is the driving force causing the box to expand when zero pressure is applied and the starting configuration has near zero forces on all of the atoms?
My LAMMPS version is 29 Oct 2020.
# Reax/C potential
units real
atom_style charge
dimension 3
boundary p p p
read_data data.CrSi2
replicate 3 3 3
set group all charge 0.0
pair_style reax/c NULL
pair_coeff * * ffield.reax.0 Cr Si
neighbor 2.0 bin
neigh_modify every 1 delay 0 check yes
timestep 0.2
thermo_modify norm yes
thermo 50
dump coords all custom 10 test.lammpstrj id type q x y z fx fy fz
dump_modify coords sort id
velocity all set 0.0 0.0 0.0
fix 0 all qeq/reax 1 0.0 10.0 1.0e-4 reax/c
#minimize 1e-8 0.0 10000 100000
variable a loop 10
#fix 1 all box/relax tri 0.0 scalexy no scalexz no scaleyz no
fix 1 all box/relax iso 0.0
label loop
minimize 1e-8 0.0 100000 1000000
next a
jump SELF loop