# fix nve/bpm/sphere command

## Syntax

    fix ID group-ID nve/bpm/sphere

-   ID, group-ID are documented in [fix](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

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

## Description

::: versionadded
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](fix_nve) command, which assumes
point particles and only updates their position and velocity. It also
differs from the [fix nve/sphere](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](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](restart). None of the [fix_modify](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](Howto_output). No
parameter of this fix can be used with the *start/stop* keywords of the
[run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

This fix is part of the BPM package. It is only enabled if LAMMPS was
built with that package. See the [Build package](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](atom_style) 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](dimension) keyword.

## Related commands

[fix nve](fix_nve), [fix nve/sphere](fix_nve_sphere)

## Default

none