forked from lijiext/lammps
211 lines
8.8 KiB
Plaintext
211 lines
8.8 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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velocity command :h3
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[Syntax:]
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velocity group-ID style args keyword args ... :pre
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group-ID = ID of group of atoms whose velocity will be changed :ulb,l
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style = {create} or {set} or {scale} or {ramp} or {zero} :l
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{create} 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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{set} 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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{scale} arg = temp
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temp = temperature value (temperature units)
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{ramp} args = vdim vlo vhi dim clo chi
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vdim = {vx} or {vy} or {vz}
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vlo,vhi = lower and upper velocity value (velocity units)
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dim = {x} or {y} or {z}
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clo,chi = lower and upper coordinate bound (distance units)
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{zero} arg = {linear} or {angular}
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{linear} = zero the linear momentum
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{angular} = zero the angular momentum :pre
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zero or more keyword/value pairs may be appended :l
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keyword = {dist} or {sum} or {mom} or {rot} or {temp} or {loop} or {units} :l
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{dist} value = {uniform} or {gaussian}
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{sum} value = {no} or {yes}
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{mom} value = {no} or {yes}
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{rot} value = {no} or {yes}
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{temp} value = temperature ID
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{loop} value = {all} or {local} or {geom}
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{rigid} value = fix-ID
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fix-ID = ID of rigid body fix
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{units} value = {box} or {lattice} :pre
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:ule
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[Examples:]
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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 :pre
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[Description:]
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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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The {create} 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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The {set} 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 "variable"_variable.html. 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.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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keywords for the simulation box parameters or other parameters.
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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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The {scale} 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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The {ramp} style is similar to that used by the "compute
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temp/ramp"_compute_temp_ramp.html 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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The {zero} 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 {rigid} 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 "fix rigid or fix
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rigid/small"_fix_rigid.html 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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All temperatures specified in the velocity command are in temperature
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units; see the "units"_units.html command. The units of velocities and
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coordinates depend on whether the {units} keyword is set to {box} or
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{lattice}, as discussed below.
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For all styles, no atoms are assigned z-component velocities if the
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simulation is 2d; see the "dimension"_dimension.html command.
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:line
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The keyword/value option pairs are used in the following ways by the
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various styles.
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The {dist} option is used by {create}. The ensemble of generated
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velocities can be a {uniform} 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 {gaussian} distribution with a mean of 0.0 and a sigma scaled to
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produce the requested temperature.
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The {sum} option is used by all styles, except {zero}. 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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The {mom} and {rot} options are used by {create}. 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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The {temp} option is used by {create} and {scale} to specify a
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"compute"_compute.html that calculates temperature in a desired way.
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If this option is not specified, {create} and {scale} calculate
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temperature using a compute that is defined as follows:
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compute velocity_temp group-ID temp :pre
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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 "compute
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temp"_compute_temp.html 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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:line
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The {loop} option is used by {create} in the following ways.
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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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"create_atoms"_create_atoms.html command, since atom IDs will likely
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be assigned to atoms differently.
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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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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 {all} or
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{local} options are used.
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Note that the {loop geom} 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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:line
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The {rigid} option only has meaning when used with the {zero} style.
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It allows specification of a fix-ID for one of the "rigid-body
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fix"_fix_rigid.html 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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The {units} option is used by {set} and {ramp}. 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 "units"_units.html 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 "lattice"_lattice.html
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command must have been previously used to define the lattice spacing.
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[Restrictions:] none
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[Related commands:]
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"fix shake"_fix_shake.html, "lattice"_lattice.html
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[Default:]
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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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