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### fix nve/limit command

Syntax:

    fix ID group-ID nve/limit limitstyle xmax

- ID, group-ID are documented in fix command
- nve/limit = style name of this fix command
- limitstyle = absolute or radius_ratio
- xmax = maximum distance an atom can move in one timestep (distance units or relative to atom radius)

Examples:

    fix 1 all nve/limit absolute 0.1

Description:

Perform constant NVE updates of position and velocity for atoms in the group each timestep. A limit is imposed on the maximum distance an atom can move in one timestep. This is useful when starting a simulation with a configuration containing highly overlapped atoms. Normally this would generate huge forces which would blow atoms out of the simulation box, causing LAMMPS to stop with an error.

Using this fix can overcome that problem. Forces on atoms must still be computable (which typically means 2 atoms must have a separation distance > 0.0). But large velocities generated by large forces are reset to a value that corresponds to a displacement of length xmax in a single timestep. Xmax is specified in distance units; see the units command for details. The value of xmax should be consistent with the neighbor skin distance and the frequency of neighbor list re-building, so that pairwise interactions are not missed on successive timesteps as atoms move. See the neighbor and neigh_modify commands for details.

If limitstyle absolute is used, xmax is applied directly. If limitstyle radius_ratio is used, a maxmimum distance per step of xmax*radius is applied for each atom. This can be useful for the simulation of poly-disperse systems. Note that this option requires the atom radius to be stored by using an appropriate atom style.

Note that if a velocity reset occurs the integrator will not conserve energy. On steps where no velocity resets occur, this integrator is exactly like the fix nve command. Since forces are unaltered, pressures computed by thermodynamic output will still be very large for overlapped configurations.

IMPORTANT NOTE: You should not use fix shake in conjunction with this fix. That is because fix shake applies contraint forces based on the predicted postitions of atoms after the next timestep. It has no way of knowing the timestep may change due to this fix, which will cause the constraint forces to be invalid. A better strategy is to turn off fix shake when performing initial dynamics that need this fix, then turn fix shake on when doing normal dynamics with a fixed-size timestep.

Restart, fix_modify, output, run start/stop, minimize info:

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

This fix computes a global scalar which can be accessed by various output commands. The scalar is the count of how many updates of atom's velocity/position were limited by the maximum distance criterion. This should be roughly the number of atoms so affected, except that updates occur at both the beginning and end of a timestep in a velocity Verlet timestepping algorithm. This is a cumulative quantity for the current run, but is re-initialized to zero each time a run is performed. The scalar value calculated by this fix is "extensive".

No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions: none

Related commands:

fix nve, fix nve/noforce, pair_style soft

Default: none