forked from lijiext/lammps
223 lines
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HTML
223 lines
9.9 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>velocity command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>velocity group-ID style args keyword value ...
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</PRE>
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<UL><LI>group-ID = ID of group of atoms whose velocity will be changed
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<LI>style = <I>create</I> or <I>set</I> or <I>scale</I> or <I>ramp</I> or <I>zero</I>
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<PRE> <I>create</I> args = temp seed
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temp = temperature value (temperature units)
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seed = random # seed (positive integer)
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<I>set</I> args = vx vy vz
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vx,vy,vz = velocity value or NULL (velocity units)
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any of vx,vy,vz van be a variable (see below)
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<I>scale</I> arg = temp
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temp = temperature value (temperature units)
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<I>ramp</I> args = vdim vlo vhi dim clo chi
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vdim = <I>vx</I> or <I>vy</I> or <I>vz</I>
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vlo,vhi = lower and upper velocity value (velocity units)
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dim = <I>x</I> or <I>y</I> or <I>z</I>
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clo,chi = lower and upper coordinate bound (distance units)
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<I>zero</I> arg = <I>linear</I> or <I>angular</I>
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<I>linear</I> = zero the linear momentum
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<I>angular</I> = zero the angular momentum
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</PRE>
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>dist</I> or <I>sum</I> or <I>mom</I> or <I>rot</I> or <I>temp</I> or <I>loop</I> or <I>units</I>
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<PRE> <I>dist</I> value = <I>uniform</I> or <I>gaussian</I>
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<I>sum</I> value = <I>no</I> or <I>yes</I>
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<I>mom</I> value = <I>no</I> or <I>yes</I>
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<I>rot</I> value = <I>no</I> or <I>yes</I>
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<I>temp</I> value = temperature ID
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<I>loop</I> value = <I>all</I> or <I>local</I> or <I>geom</I>
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<I>rigid</I> value = fix-ID
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fix-ID = ID of rigid body fix
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<I>units</I> value = <I>box</I> or <I>lattice</I>
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>velocity all create 300.0 4928459 rot yes dist gaussian
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velocity border set NULL 4.0 v_vz sum yes units box
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velocity flow scale 300.0
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velocity flow ramp vx 0.0 5.0 y 5 25 temp mytemp
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velocity all zero linear
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Set or change the velocities of a group of atoms in one of several
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styles. For each style, there are required arguments and optional
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keyword/value parameters. Not all options are used by each style.
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Each option has a default as listed below.
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</P>
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<P>The <I>create</I> style generates an ensemble of velocities using a random
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number generator with the specified seed as the specified temperature.
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</P>
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<P>The <I>set</I> style sets the velocities of all atoms in the group to the
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specified values. If any component is specified as NULL, then it is
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not set. Any of the vx,vy,vz velocity components can be specified as
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an equal-style or atom-style <A HREF = "variable.html">variable</A>. If the value
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is a variable, it should be specified as v_name, where name is the
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variable name. In this case, the variable will be evaluated, and its
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value used to determine the velocity component. Note that if a
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variable is used, the velocity it calculates must be in box units, not
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lattice units; see the discussion of the <I>units</I> keyword below.
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</P>
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<P>Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
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keywords for the simulation box parameters or other parameters.
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</P>
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<P>Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent
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velocity field.
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</P>
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<P>The <I>scale</I> style computes the current temperature of the group of
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atoms and then rescales the velocities to the specified temperature.
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</P>
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<P>The <I>ramp</I> style is similar to that used by the <A HREF = "compute_temp_ramp.html">compute
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temp/ramp</A> command. Velocities ramped
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uniformly from vlo to vhi are applied to dimension vx, or vy, or vz.
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The value assigned to a particular atom depends on its relative
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coordinate value (in dim) from clo to chi. For the example above, an
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atom with y-coordinate of 10 (1/4 of the way from 5 to 25), would be
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assigned a x-velocity of 1.25 (1/4 of the way from 0.0 to 5.0). Atoms
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outside the coordinate bounds (less than 5 or greater than 25 in this
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case), are assigned velocities equal to vlo or vhi (0.0 or 5.0 in this
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case).
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</P>
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<P>The <I>zero</I> style adjusts the velocities of the group of atoms so that
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the aggregate linear or angular momentum is zero. No other changes
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are made to the velocities of the atoms. If the <I>rigid</I> option is
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specified (see below), then the zeroing is performed on individual
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rigid bodies, as defined by the <A HREF = "fix_rigid.html">fix rigid or fix
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rigid/small</A> commands. In other words, zero linear
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will set the linear momentum of each rigid body to zero, and zero
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angular will set the angular momentum of each rigid body to zero.
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This is done by adjusting the velocities of the atoms in each rigid
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body.
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</P>
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<P>All temperatures specified in the velocity command are in temperature
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units; see the <A HREF = "units.html">units</A> command. The units of velocities and
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coordinates depend on whether the <I>units</I> keyword is set to <I>box</I> or
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<I>lattice</I>, as discussed below.
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</P>
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<P>For all styles, no atoms are assigned z-component velocities if the
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simulation is 2d; see the <A HREF = "dimension.html">dimension</A> command.
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</P>
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<HR>
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<P>The keyword/value option pairs are used in the following ways by the
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various styles.
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</P>
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<P>The <I>dist</I> option is used by <I>create</I>. The ensemble of generated
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velocities can be a <I>uniform</I> distribution from some minimum to
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maximum value, scaled to produce the requested temperature. Or it can
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be a <I>gaussian</I> distribution with a mean of 0.0 and a sigma scaled to
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produce the requested temperature.
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</P>
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<P>The <I>sum</I> option is used by all styles, except <I>zero</I>. The new
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velocities will be added to the existing ones if sum = yes, or will
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replace them if sum = no.
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</P>
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<P>The <I>mom</I> and <I>rot</I> options are used by <I>create</I>. If mom = yes, the
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linear momentum of the newly created ensemble of velocities is zeroed;
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if rot = yes, the angular momentum is zeroed.
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</P>
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<P>The <I>temp</I> option is used by <I>create</I> and <I>scale</I> to specify a
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<A HREF = "compute.html">compute</A> that calculates temperature in a desired way.
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If this option is not specified, <I>create</I> and <I>scale</I> calculate
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temperature using a compute that is defined as follows:
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</P>
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<PRE>compute velocity_temp group-ID temp
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</PRE>
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<P>where group-ID is the same ID used in the velocity command. i.e. the
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group of atoms whose velocity is being altered. This compute is
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deleted when the velocity command is finished. See the <A HREF = "compute_temp.html">compute
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temp</A> command for details. If the computed
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temperature should have degrees-of-freedom removed due to fix
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constraints (e.g. SHAKE or rigid-body constraints), then the
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appropriate fix command must be specified before the velocity command
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is issued.
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</P>
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<HR>
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<P>The <I>loop</I> option is used by <I>create</I> in the following ways.
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</P>
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<P>If loop = all, then each processor loops over all atoms in the
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simulation to create velocities, but only stores velocities for atoms
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it owns. This can be a slow loop for a large simulation. If atoms
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were read from a data file, the velocity assigned to a particular atom
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will be the same, independent of how many processors are being used.
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This will not be the case if atoms were created using the
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<A HREF = "create_atoms.html">create_atoms</A> command, since atom IDs will likely
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be assigned to atoms differently.
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</P>
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<P>If loop = local, then each processor loops over only its atoms to
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produce velocities. The random number seed is adjusted to give a
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different set of velocities on each processor. This is a fast loop,
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but the velocity assigned to a particular atom will depend on which
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processor owns it. Thus the results will always be different when a
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simulation is run on a different number of processors.
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</P>
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<P>If loop = geom, then each processor loops over only its atoms. For
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each atom a unique random number seed is created, based on the atom's
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xyz coordinates. A velocity is generated using that seed. This is a
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fast loop and the velocity assigned to a particular atom will be the
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same, independent of how many processors are used. However, the set
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of generated velocities may be more correlated than if the <I>all</I> or
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<I>local</I> options are used.
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</P>
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<P>Note that the <I>loop geom</I> option will not necessarily assign identical
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velocities for two simulations run on different machines. This is
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because the computations based on xyz coordinates are sensitive to
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tiny differences in the double-precision value for a coordinate as
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stored on a particular machine.
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</P>
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<HR>
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<P>The <I>rigid</I> option only has meaning when used with the <I>zero</I> style.
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It allows specification of a fix-ID for one of the <A HREF = "fix_rigid.html">rigid-body
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fix</A> variants which defines a set of rigid bodies. The
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zeroing of linear or angular momentum is then performed for each rigid
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body defined by the fix, as described above.
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</P>
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<P>The <I>units</I> option is used by <I>set</I> and <I>ramp</I>. If units = box,
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the velocities and coordinates specified in the velocity command are
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in the standard units described by the <A HREF = "units.html">units</A> command
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(e.g. Angstroms/fmsec for real units). If units = lattice, velocities
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are in units of lattice spacings per time (e.g. spacings/fmsec) and
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coordinates are in lattice spacings. The <A HREF = "lattice.html">lattice</A>
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command must have been previously used to define the lattice spacing.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_shake.html">fix shake</A>, <A HREF = "lattice.html">lattice</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are dist = uniform, sum = no, mom = yes, rot = no,
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temp = full style on group-ID, loop = all, and units = lattice. The
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rigid option is not defined by default.
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</P>
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</HTML>
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