# velocity command

## Syntax

    velocity group-ID style args keyword value ...

- group-ID = ID of group of atoms whose velocity will be changed
- style = set or ramp or zero

    set args = vx vy vz
      vx,vy,vz = velocity value or NULL (velocity units)
      any of vx,vy,vz van be a variable (see below)
    setAngular args = angularmx angularmy angularmz
      angularmx,angularmy,angularmz = angular momentum value or NULL (angular mometum units)
      any of angularmx,angularmy,angularmz van be a variable (see below)
    ramp args = vdim vlo vhi dim clo chi
      vdim = vx or vy or vz
      vlo,vhi = lower and upper velocity value (velocity units)
      dim = x or y or z
      clo,chi = lower and upper coordinate bound (distance units)
    zero arg = linear or angular
      linear = zero the linear momentum
      angular = zero the angular momentum

- zero or more keyword/value pairs may be appended
- keyword = dist or sum or mom or rot or loop or units

    dist value = uniform or gaussian
    sum value = no or yes
    mom value = no or yes
    rot value = no or yes
    loop value = all or local or geom
    rigid value = fix-ID
      fix-ID = ID of rigid body fix
    units value = box or lattice

## Examples

    velocity border set NULL 4.0 v_vz sum yes units box
    velocity all zero linear

## Description

Set or change the velocities of a group of atoms in one of several styles. For each style, there are required arguments and optional keyword/value parameters. Not all options are used by each style. Each option has a default as listed below.

The set style sets the velocities of all atoms in the group to the specified values. If any component is specified as NULL, then it is not set. Any of the vx,vy,vz velocity components can be specified as an equal-style or atom-style variable. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated, and its value used to determine the velocity component.

The setAngular is simular to set, however, sets the angular momentum.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters or other parameters.

Atom-style variables can specify the same formulas as equal-style variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent velocity field.

The ramp style makekes velocities ramped uniformly from vlo to vhi are applied to dimension vx, or vy, or vz. The value assigned to a particular atom depends on its relative coordinate value (in dim) from clo to chi. For the example above, an atom with y-coordinate of 10 (1/4 of the way from 5 to 25), would be assigned a x-velocity of 1.25 (1/4 of the way from 0.0 to 5.0). Atoms outside the coordinate bounds (less than 5 or greater than 25 in this case), are assigned velocities equal to vlo or vhi (0.0 or 5.0 in this case).

The zero style adjusts the velocities of the group of atoms so that the aggregate linear or angular momentum is zero. No other changes are made to the velocities of the atoms. If the rigid option is specified (see below), then the zeroing is performed on individual rigid bodies, as defined by the fix rigid or fix rigid/small commands. In other words, zero linear will set the linear momentum of each rigid body to zero, and zero angular will set the angular momentum of each rigid body to zero. This is done by adjusting the velocities of the atoms in each rigid body.

For all styles, no atoms are assigned z-component velocities if the simulation is 2d; see the dimension command.

The keyword/value option pairs are used in the following ways by the various styles.

The sum option is used by all styles, except zero. The new velocities will be added to the existing ones if sum = yes, or will replace them if sum = no.

The mom and rot options are used by create. If mom = yes, the linear momentum of the newly created ensemble of velocities is zeroed; if rot = yes, the angular momentum is zeroed.

The loop option is used by create in the following ways.

If loop = all, then each processor loops over all atoms in the simulation to create velocities, but only stores velocities for atoms it owns. This can be a slow loop for a large simulation. If atoms were read from a data file, the velocity assigned to a particular atom will be the same, independent of how many processors are being used. This will not be the case if atoms were created using the create_atoms command, since atom IDs will likely be assigned to atoms differently.

If loop = local, then each processor loops over only its atoms to produce velocities. The random number seed is adjusted to give a different set of velocities on each processor. This is a fast loop, but the velocity assigned to a particular atom will depend on which processor owns it. Thus the results will always be different when a simulation is run on a different number of processors.

If loop = geom, then each processor loops over only its atoms. For each atom a unique random number seed is created, based on the atom’s xyz coordinates. A velocity is generated using that seed. This is a fast loop and the velocity assigned to a particular atom will be the same, independent of how many processors are used. However, the set of generated velocities may be more correlated than if the all or local options are used.

Note that the loop geom option will not necessarily assign identical velocities for two simulations run on different machines. This is because the computations based on xyz coordinates are sensitive to tiny differences in the double-precision value for a coordinate as stored on a particular machine.

The rigid option only has meaning when used with the zero style. It allows specification of a fix-ID for one of the rigid-body fix variants which defines a set of rigid bodies. The zeroing of linear or angular momentum is then performed for each rigid body defined by the fix, as described above.

The units option is used by set, setAngular and ramp. If units = box, the velocities and coordinates specified in the velocity command are in the standard units described by the units command (e.g. Angstroms/fmsec for real units). If units = lattice, velocities are in units of lattice spacings per time (e.g. spacings/fmsec) and coordinates are in lattice spacings. The lattice command must have been previously used to define the lattice spacing.

## Restrictions

> none

## Related commands

`fix shake`, lattice

## Default

The option defaults are dist = uniform, sum = no, mom = yes, rot = no,
  style on group-ID, loop = all, and units = box. The

rigid option is not defined by default.