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# velocity command

## Syntax

    velocity group-ID style args keyword value ...

- group-ID = ID of group of atoms whose velocity will be changed

- style = create or set or scale or ramp or zero

      create args = temp seed
        temp = temperature value (temperature units)
        seed = random # seed (positive integer)
      set args = vx vy vz
        vx,vy,vz = velocity value or NULL (velocity units)
        any of vx,vy,vz can be a variable (see below)
      scale arg = temp
        temp = temperature value (temperature units)
      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 temp or bias or loop or rigid or units

      dist value = uniform or gaussian
      sum value = no or yes
      mom value = no or yes
      rot value = no or yes
      temp value = temperature compute ID
      bias 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 all create 300.0 4928459 rot yes dist gaussian
    velocity border set NULL 4.0 v_vz sum yes units box
    velocity flow scale 300.0
    velocity flow ramp vx 0.0 5.0 y 5 25 temp mytemp
    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 create style generates an ensemble of velocities using a random number generator with the specified seed at the specified temperature.

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. Note that if a variable is used, the velocity it calculates must be in box units, not lattice units; see the discussion of the units keyword below.

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 scale style computes the current temperature of the group of atoms and then rescales the velocities to the specified temperature.

The ramp style is similar to that used by the compute temp/ramp command. 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.

All temperatures specified in the velocity command are in temperature units; see the units command. The units of velocities and coordinates depend on whether the units keyword is set to box or lattice, as discussed below.

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

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

The dist keyword is used by create. The ensemble of generated velocities can be a uniform distribution from some minimum to maximum value, scaled to produce the requested temperature. Or it can be a gaussian distribution with a mean of 0.0 and a sigma scaled to produce the requested temperature.

The sum keyword 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 keywords 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.

If specified, the temp keyword is used by create and scale to specify a compute that calculates temperature in a desired way, e.g. by first subtracting out a velocity bias, as discussed on the Howto thermostat doc page. If this keyword is not specified, create and scale calculate temperature using a compute that is defined internally as follows:

    compute velocity_temp group-ID temp

where group-ID is the same ID used in the velocity command. i.e. the group of atoms whose velocity is being altered. This compute is deleted when the velocity command is finished. See the compute temp command for details. If the calculated temperature should have degrees-of-freedom removed due to fix constraints (e.g. SHAKE or rigid-body constraints), then the appropriate fix command must be specified before the velocity command is issued.

The bias keyword with a yes setting is used by create and scale, but only if the temp keyword is also used to specify a compute that calculates temperature in a desired way. If the temperature compute also calculates a velocity bias, the bias is subtracted from atom velocities before the create and scale operations are performed. After the operations, the bias is added back to the atom velocities. See the Howto thermostat page for more discussion of temperature computes with biases. Note that the velocity bias is only applied to atoms in the temperature compute specified with the temp keyword.

As an example, assume atoms are currently streaming in a flow direction (which could be separately initialized with the ramp style), and you wish to initialize their thermal velocity to a desired temperature. In this context thermal velocity means the per-particle velocity that remains when the streaming velocity is subtracted. This can be done using the create style with the temp keyword specifying the ID of a compute temp/ramp or compute temp/profile command, and the bias keyword set to a yes value.

The loop keyword 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 keywords are used.

Note that the loop geom keyword 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 keyword 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 keyword is used by set 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/fs for real units). If units = lattice, velocities are in units of lattice spacings per time (e.g. spacings/fs) and coordinates are in lattice spacings. The lattice command must have been previously used to define the lattice spacing.

## Restrictions

Assigning a temperature via the create style to a system with rigid bodies or SHAKE constraints may not have the desired outcome for two reasons. First, the velocity command can be invoked before all of the relevant fixes are created and initialized and the number of adjusted degrees of freedom (DOFs) is known. Thus it is not possible to compute the target temperature correctly. Second, the assigned velocities may be partially canceled when constraints are first enforced, leading to a different temperature than desired. A workaround for this is to perform a run 0 command, which ensures all DOFs are accounted for properly, and then rescale the temperature to the desired value before performing a simulation. For example:

    velocity all create 300.0 12345
    run 0                             # temperature may not be 300K
    velocity all scale 300.0          # now it should be

## Related commands

fix rigid, fix shake, lattice

## Default

The keyword defaults are dist = uniform, sum = no, mom = yes, rot = no, bias = no, loop = all, and units = lattice. The temp and rigid keywords are not defined by default.