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151 lines
7.3 KiB
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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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<H3>compute command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>compute ID group-ID style args
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</PRE>
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<UL><LI>ID = user-assigned name for the computation
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<LI>group-ID = ID of the group of atoms to perform the computation on
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<LI>style = one of a list of possible style names (see below)
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<LI>args = arguments used by a particular style
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>compute 1 all temp
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compute newtemp flow temp/partial 1 1 0
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compute 3 all ke/atom
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Create a computation that will be performed on a group of atoms.
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Quantities calculated by a compute are instantaneous values, meaning
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they are calculated from information about atoms on the current
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timestep or iteration. There are two kinds of computes, "global"
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computes that calculate one or more values for the entire group of
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atoms, and "per-atom" computes that calculate one or more values for
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each atom in the group. The latter has the word "atom" in its style
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name.
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</P>
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<P>In LAMMPS, a "compute" can be used in several ways. The results of
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global computes can be output via the <A HREF = "thermo_style.html">thermo_style
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custom</A> or <A HREF = "fix_ave_time.html">fix ave/time</A> command.
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Or the values can be referenced in a <A HREF = "variable.html">variable equal</A> or
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<A HREF = "variable.html">variable atom</A> command. The results of computes that
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calculate a global temperature or pressure can be used by fixes that
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do thermostatting or barostatting and when atom velocities are
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created.
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</P>
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<P>The results of per-atom computes can be output via the <A HREF = "dump.html">dump
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custom</A> command or the <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A> command. Or the per-atom values can
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be time-averaged via the <A HREF = "fix_ave_atom.html">fix ave/atom</A> command and
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then output via the <A HREF = "dump.html">dump custom</A> or <A HREF = "fix_ave_spatial.html">fix
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ave/spatial</A> commands. Or the per-atom values
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can be referenced in a <A HREF = "variable.html">variable atom</A> command. Note
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that the value of per-atom computes will be 0.0 for atoms not in the
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specified compute group.
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</P>
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<P>See this <A HREF = "Section_howto.html#4_15">howto section</A> for a summary of
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various LAMMPS output options, many of which involve computes.
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</P>
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<P>The ID of a compute can only contain alphanumeric characters and
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underscores.
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</P>
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<P>The results of computes that calculate global quantities can be either
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"intensive" or "extensive" values. Intensive means the value is
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independent of the number of atoms in the simulation,
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e.g. temperature. Extensive means the value scales with the number of
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atoms in the simulation, e.g. total rotational kinetic energy.
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<A HREF = "thermo_style.html">Thermodynamic output</A> will normalize extensive
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values depending on the "thermo_modify norm" setting. But if a
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compute value is accessed in another way, e.g. by a
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<A HREF = "variable.html">variable</A>, you may need to know whether it is an
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intensive or extensive value. See the doc page for individual
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computes for further info.
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</P>
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<P>LAMMPS creates its own global computes for thermodynamic output.
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Three computes are always created, named "thermo_temp",
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"thermo_press", and "thermo_pe", as if these commands had been invoked
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in the input script:
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</P>
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<PRE>compute thermo_temp all temp
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compute thermo_press all pressure thermo_temp
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compute thermo_pe all pe
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</PRE>
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<P>Additional computes for other quantities are created if the thermo
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style requires it. See the documentation for the
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<A HREF = "thermo_style.html">thermo_style</A> command.
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</P>
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<P>Fixes that calculate temperature or pressure, i.e. for thermostatting
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or barostatting, may also create computes. These are discussed in the
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documentation for specific <A HREF = "fix.html">fix</A> commands.
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</P>
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<P>In all these cases, the default computes can be replaced by computes
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defined by the user in the input script, as described by the
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<A HREF = "thermo_modify.html">thermo_modify</A> and <A HREF = "fix_modify.html">fix modify</A>
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commands.
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</P>
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<P>Properties of either a default or user-defined compute can be modified
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via the <A HREF = "compute_modify.html">compute_modify</A> command.
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</P>
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<P>Computes can be deleted with the <A HREF = "uncompute.html">uncompute</A> command.
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</P>
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<P>Code for new computes can be added to LAMMPS (see <A HREF = "Section_modify.html">this
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section</A> of the manual) and the results of their
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calculations accessed in the various ways described above.
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</P>
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<P>Each compute style has its own doc page which describes its arguments
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and what it does. Here is an alphabetic list of compute styles
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available in LAMMPS:
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</P>
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<UL><LI><A HREF = "compute_centro_atom.html">centro/atom</A> - centro-symmetry parameter for each atom
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<LI><A HREF = "compute_coord_atom.html">coord/atom</A> - coordination number for each atom
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<LI><A HREF = "compute_damage_atom.html">damage/atom</A> - Peridynamic damage for each atom
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<LI><A HREF = "compute_displace_atom.html">displace/atom</A> - displacement of each atom
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<LI><A HREF = "compute_erotate_asphere.html">erotate/asphere</A> - rotational energy of aspherical particles
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<LI><A HREF = "compute_erotate_sphere.html">erotate/sphere</A> - rotational energy of spherical particles
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<LI><A HREF = "compute_group_group.html">group/group</A> - energy/force between two groups of atoms
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<LI><A HREF = "compute_ke.html">ke</A> - translational kinetic energy
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<LI><A HREF = "compute_ke_atom.html">ke/atom</A> - kinetic energy for each atom
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<LI><A HREF = "compute_pe.html">pe</A> - potential energy
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<LI><A HREF = "compute_pe_atom.html">pe/atom</A> - potential energy for each atom
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<LI><A HREF = "compute_pressure.html">pressure</A> - total pressure and pressure tensor
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<LI><A HREF = "compute_reduce.html">reduce</A> - combine per-atom quantities into a single global value
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<LI><A HREF = "compute_stress_atom.html">stress/atom</A> - stress tensor for each atom
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<LI><A HREF = "compute_temp.html">temp</A> - temperature of group of atoms
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<LI><A HREF = "compute_temp_asphere.html">temp/asphere</A> - temperature of aspherical particles
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<LI><A HREF = "compute_temp_com.html">temp/com</A> - temperature after subtracting center-of-mass velocity
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<LI><A HREF = "compute_temp_deform.html">temp/deform</A> - temperature excluding box deformation velocity
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<LI><A HREF = "compute_temp_partial.html">temp/partial</A> - temperature excluding one or more dimensions of velocity
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<LI><A HREF = "compute_temp_ramp.html">temp/ramp</A> - temperature excluding ramped velocity component
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<LI><A HREF = "compute_temp_region.html">temp/region</A> - temperature of a region of atoms
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<LI><A HREF = "compute_temp_sphere.html">temp/sphere</A> - temperature of spherical particles
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</UL>
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<P>There are also additional compute styles submitted by users which are
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included in the LAMMPS distribution. The list of these with links to
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the individual styles are given in the compute section of <A HREF = "Section_commands.html#3_5">this
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page</A>.
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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 = "uncompute.html">uncompute</A>, <A HREF = "compute_modify.html">compute_modify</A>, <A HREF = "fix_ave_atom.html">fix
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ave/atom</A>, <A HREF = "fix_ave_spatial.html">fix ave/spatial</A>,
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<A HREF = "fix_ave_time.html">fix ave/time</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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