forked from lijiext/lammps
122 lines
5.2 KiB
HTML
122 lines
5.2 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>compute temp/ramp 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 temp/ramp vdim vlo vhi dim clo chi keyword value ...
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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>temp/ramp = style name of this compute command
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<LI>vdim = <I>vx</I> or <I>vy</I> or <I>vz</I>
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<LI>vlo,vhi = subtract velocities between vlo and vhi (velocity units)
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<LI>dim = <I>x</I> or <I>y</I> or <I>z</I>
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<LI>clo,chi = lower and upper bound of domain to subtract from (distance units)
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>units</I>
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</UL>
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<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>compute 2nd middle temp/ramp vx 0 8 y 2 12 units lattice
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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 computation that calculates the temperature of a group of
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atoms, after subtracting out an ramped velocity profile before
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computing the kinetic energy. A compute of this style can be used by
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any command that computes a temperature,
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e.g. <A HREF = "thermo_modify.html">thermo_modify</A>, <A HREF = "fix_temp_rescale.html">fix
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temp/rescale</A>, <A HREF = "fix_npt.html">fix npt</A>, etc.
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</P>
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<P>The meaning of the arguments for this command which define the
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velocity ramp are the same as for the <A HREF = "velocity.html">velocity ramp</A>
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command which was presumably used to impose the velocity.
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</P>
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<P>After the ramp velocity has been subtracted from the specified
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dimension for each atom, the temperature is calculated by the formula
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KE = dim/2 N k T, where KE = total kinetic energy of the group of
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atoms (sum of 1/2 m v^2), dim = 2 or 3 = dimensionality of the
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simulation, N = number of atoms in the group, k = Boltzmann constant,
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and T = temperature.
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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for coordinates (c1,c2) and velocities (vlo,vhi). A <I>box</I> value
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selects standard distance units as defined by the <A HREF = "units.html">units</A>
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command, e.g. Angstroms for units = real or metal. A <I>lattice</I> value
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means the distance units are in lattice spacings; e.g. velocity =
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lattice spacings / tau. The <A HREF = "lattice.html">lattice</A> command must have
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been previously used to define the lattice spacing.
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</P>
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<P>A kinetic energy tensor, stored as a 6-element vector, is also
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calculated by this compute for use in the computation of a pressure
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tensor. The formula for the components of the tensor is the same as
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the above formula, except that v^2 is replaced by vx*vy for the xy
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component, etc. The 6 components of the vector are ordered xx, yy,
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zz, xy, xz, yz.
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</P>
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<P>The number of atoms contributing to the temperature is assumed to be
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constant for the duration of the run; use the <I>dynamic</I> option of the
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<A HREF = "compute_modify.html">compute_modify</A> command if this is not the case.
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</P>
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<P>The removal of the ramped velocity component by this fix is
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essentially computing the temperature after a "bias" has been removed
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from the velocity of the atoms. If this compute is used with a fix
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command that performs thermostatting then this bias will be subtracted
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from each atom, thermostatting of the remaining thermal velocity will
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be performed, and the bias will be added back in. Thermostatting
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fixes that work in this way include <A HREF = "fix_nvt.html">fix nvt</A>, <A HREF = "fix_temp_rescale.html">fix
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temp/rescale</A>, <A HREF = "fix_temp_berendsen">fix
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temp/berendsen</A>, and <A HREF = "fix_langevin.html">fix
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langevin</A>.
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</P>
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<P>This compute subtracts out degrees-of-freedom due to fixes that
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constrain molecular motion, such as <A HREF = "fix_shake.html">fix shake</A> and
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<A HREF = "fix_rigid.html">fix rigid</A>. This means the temperature of groups of
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atoms that include these constraints will be computed correctly. If
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needed, the subtracted degrees-of-freedom can be altered using the
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<I>extra</I> option of the <A HREF = "compute_modify.html">compute_modify</A> command.
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</P>
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<P>See <A HREF = "Section_howto.html#4_16">this howto section</A> of the manual for a
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discussion of different ways to compute temperature and perform
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thermostatting.
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</P>
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<P><B>Output info:</B>
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</P>
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<P>This compute calculates a global scalar (the temperature) and a global
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vector of length 6 (KE tensor), which can be accessed by indices 1-6.
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These values can be used by any command that uses global scalar or
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vector values from a compute as input. See <A HREF = "Section_howto.html#4_15">this
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section</A> for an overview of LAMMPS output
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options.
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</P>
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<P>The scalar value calculated by this compute is "intensive", meaning it
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is independent of the number of atoms in the simulation. The vector
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values are "extensive", meaning they scale with the number of atoms in
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the 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_temp.html">compute temp</A>, <A HREF = "compute_temp_profile.html">compute
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temp/profie</A>, <A HREF = "compute_temp_deform.html">compute
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temp/deform</A>, <A HREF = "compute_pressure.html">compute
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pressure</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 default is units = lattice.
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</P>
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</HTML>
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