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.
@@ -121,30 +126,41 @@ constraints (e.g. SHAKE or rigid-body constraints), then the appropriate fix command must be specified before the velocity command is issued. -The loop option is used by create. 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. It will produce the same set of velocities, -independent of the number of processors, if atoms were read from a -data file. It will not produce such independent velocities if atoms -were created using the create_atoms command. 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 will -always produce different sets of velocities 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 will -always give the same set of velocities, independent of how many -processors are used. However, the 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 +
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, regardless 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 will always give the same set of velocities, independent +of how many processors are used. However, the 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 units option 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 @@ -153,9 +169,6 @@ 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.
-For all styles, no atoms are assigned z-component velocities if the -simulation is 2d; see the dimension command. -
Restrictions: none
Related commands: diff --git a/doc/velocity.txt b/doc/velocity.txt index fc1ea5fbe0..a548e33797 100644 --- a/doc/velocity.txt +++ b/doc/velocity.txt @@ -86,6 +86,11 @@ units; see the "units"_units.html 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"_dimension.html command. + +:line + The keyword/value option pairs are used in the following ways by the various styles. @@ -113,30 +118,41 @@ constraints (e.g. SHAKE or rigid-body constraints), then the appropriate fix command must be specified before the velocity command is issued. -The {loop} option is used by {create}. 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. It will produce the same set of velocities, -independent of the number of processors, if atoms were read from a -data file. It will not produce such independent velocities if atoms -were created using the "create_atoms"_create_atoms.html command. 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 will -always produce different sets of velocities 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 will -always give the same set of velocities, independent of how many -processors are used. However, the 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 +:line + +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, regardless of how many processors are being used. +This will not be the case if atoms were created using the +"create_atoms"_create_atoms.html 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 will always give the same set of velocities, independent +of how many processors are used. However, the 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. +:line + The {units} option 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"_units.html command @@ -145,9 +161,6 @@ are in units of lattice spacings per time (e.g. spacings/fmsec) and coordinates are in lattice spacings. The "lattice"_lattice.html command must have been previously used to define the lattice spacing. -For all styles, no atoms are assigned z-component velocities if the -simulation is 2d; see the "dimension"_dimension.html command. - [Restrictions:] none [Related commands:]