forked from lijiext/lammps
99 lines
3.9 KiB
HTML
99 lines
3.9 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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<H3>compute sum 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 sum value1 value2 ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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<LI>sum = style name of this compute command
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<LI>one or more values can be listed
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<LI>value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID[N], f_ID, f_ID[N], v_name
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<PRE> x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
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c_ID = per-atom scalar value calculated by a compute with ID
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c_ID[N] = Nth component of per-atom vector calculated by a compute with ID
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f_ID = per-atom scalar value calculated by a fix with ID
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f_ID[N] = Nth component of per-atom vector calculated by a fix with ID
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v_name = per-atom value calculated by an atom-style variable with name
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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>compute 1 all sum c_force
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compute 2 all sum c_press<B>2</B> f_ave v_myKE
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Define a calculation that sums the results of one or more per-atom
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quantities across all atoms in the group to yield a global scalar or
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vector quantity. The resulting value(s) can be accessed by any
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command that uses global computes, e.g. the <A HREF = "therml_style.html">thermo
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custom</A> command or <A HREF = "fix_ave_time.html">fix ave/time</A>
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command or by a <A HREF = "variable.html">variable</A> command. See <A HREF = "Section_howto.html#4_15">this
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section</A> of the documentation for an overview
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of output options.
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</P>
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<P>Each listed value is summed independently. The group specified with
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the command means only atoms within the group contribute to the sum.
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Note that the value that produces the per-atom quantities may define
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its own group which affects the values it returns. For example, if a
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per-atom compute is used as a value, it will generate values of 0.0
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for atoms that are not in the group specified for that compute.
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</P>
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<P>Each listed value can be an atom attribute (position, velocity, force
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component) or can be the result of a <A HREF = "compute.html">compute</A> or
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<A HREF = "fix.html">fix</A> or the evaluation of an atom-style
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<A HREF = "variable.html">variable</A>. In the latter cases, the compute, fix, or
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variable must produce a per-atom quantity, not a global quantity.
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</P>
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<P><A HREF = "compute.html">Computes</A> that produce per-atom quantities are those
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which have the word <I>atom</I> in their style name. See the doc pages for
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individual <A HREF = "fix.html">fixes</A> to determine which ones produce per-atom
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quantities. <A HREF = "variable.html">Variables</A> of style <I>atom</I> are the only
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ones that can be used with this compute since all other variable
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styles produce global quantities.
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</P>
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<P>If a single value is specified this compute produces a global scalar
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value. If multiple values are specified, this compute produces a
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vector of global values, the length of which is equal to the
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number of values specified.
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</P>
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<P>The value(s) produced by this compute are all "extensive", meaning
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their value scales linearly with the number of atoms involved. If
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normalized values are desired, this compute can be accessed by the
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<A HREF = "thermo_style.html">thermo_style custom</A> command with <A HREF = "thermo_modify.html">thermo_modify
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norm yes</A> set as an option. Or it can be accessed
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by a <A HREF = "variable.html">variable</A> that divides by the appropriate atom
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count.
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</P>
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<P><B>Output info:</B>
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</P>
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<P>The scalar and vector values calculated by this compute are
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"extensive", meaning they scale with the number of atoms in the
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simulation.
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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 = "compute.html">compute</A>, <A HREF = "fix.html">fix</A>, <A HREF = "variable.html">variable</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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