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# fix nve/bpm/sphere command

## Syntax

    fix ID group-ID nve/bpm/sphere

- ID, group-ID are documented in fix command

- nve/bpm/sphere = style name of this fix command

- zero or more keyword/value pairs may be appended

- keyword = disc

      disc value = none = treat particles as 2d discs, not spheres

## Examples

    fix 1 all nve/bpm/sphere
    fix 1 all nve/bpm/sphere disc

## Description

Added in version 4May2022.

Perform constant NVE integration to update position, velocity, angular velocity, and quaternion orientation for finite-size spherical particles in the group each timestep. V is volume; E is energy. This creates a system trajectory consistent with the microcanonical ensemble.

This fix differs from the fix nve command, which assumes point particles and only updates their position and velocity. It also differs from the fix nve/sphere command which assumes finite-size spheroid particles which do not store a quaternion. It thus does not update a particle’s orientation or quaternion.

If the disc keyword is used, then each particle is treated as a 2d disc (circle) instead of as a sphere. This is only possible for 2d simulations, as defined by the dimension keyword. The only difference between discs and spheres in this context is their moment of inertia, as used in the time integration.

## 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. No global or per-atom quantities are stored by this fix for access by various output commands. 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

This fix is part of the BPM package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

This fix requires that atoms store torque, angular velocity (omega), a radius, and a quaternion as defined by the atom_style bpm/sphere command.

All particles in the group must be finite-size spheres with quaternions. They cannot be point particles.

Use of the disc keyword is only allowed for 2d simulations, as defined by the dimension keyword.

## Related commands

fix nve, fix nve/sphere

## Default

none