Fix/restrain command: do not meet the target dihedrals

LAMMPS LAMMPS General Discussion
1

Dear all,
I use the fix/restrain command to restrain the dihedrals.
The command are following:
fix 1 all restrain dihedral 1 8 9 10 2000.0 2000.0 0.0 during npt temp 300 300 iso 1 1
in this case, the average dihedral is -2
image

However, when the target dihedrals is set 153, the command are following:
fix 1 all restrain dihedral 1 8 9 10 2000.0 2000.0 153.0 during npt temp 300 300 iso 1 1
dihedral value hard to be equilibrium
image

It should be not that the force field of my system is GAFF2 (dihedral_style fourier),
while the command in fix/restrain, the ‘dihedral’ keyword applies a dihedral restraint to the specified atoms using a simplified form of the function used by the dihedral_style charmm command

2

There is not enough information here to make any meaningful assessment.
There is no question that fix restrain will add the (harmonic) dihedral force between the selected 4 atoms, but whether this force is sufficient to enforce the dihedral’s geometry depends on which other forces act on the same atoms in the direction of the defined dihedral.