forked from lijiext/lammps
124 lines
5.0 KiB
Plaintext
Executable File
124 lines
5.0 KiB
Plaintext
Executable File
"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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compute temp/asphere command :h3
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[Syntax:]
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compute ID group-ID temp/asphere bias-ID :pre
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ID, group-ID are documented in "compute"_compute.html command
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temp/asphere = style name of this compute command
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bias-ID = ID of a temperature compute that removes a velocity bias (optional) :ul
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[Examples:]
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compute 1 all temp/asphere
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compute myTemp mobile temp/asphere tempCOM :pre
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[Description:]
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Define a computation that calculates the temperature of a group of
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aspherical particles, including a contribution from both their
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translational and rotational kinetic energy. This differs from the
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usual "compute temp"_compute_temp.html command, which assumes point
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particles with only translational kinetic energy.
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Only finite-size particles (aspherical or spherical) can be included
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in the group. For 3d finite-size particles, each has 6 degrees of
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freedom (3 translational, 3 rotational). For 2d finite-size
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particles, each has 3 degrees of freedom (2 translational, 1
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rotational).
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IMPORTANT NOTE: This choice for degrees of freedom (dof) assumes that
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all finite-size aspherical or spherical particles in your model will
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freely rotate, sampling all their rotational dof. It is possible to
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use a combination of interaction potentials and fixes that induce no
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torque or otherwise constrain some of all of your particles so that
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this is not the case. Then there are less dof and you should use the
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"compute_modify extra"_compute_modify.html command to adjust the dof
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accordingly.
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For example, an aspherical particle with all three of its
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"shape"_shape.html parameters the same is a sphere. If it does not
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rotate, then it should have 3 dof instead of 6 in 3d (or 2 instead of
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3 in 2d). A uniaxial aspherical particle has two of its three shape
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parameters the same. If it does not rotate around the axis
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perpendicular to its circular cross section, then it should have 5 dof
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instead of 6 in 3d.
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The translational kinetic energy is computed the same as is described
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by the "compute temp"_compute_temp.html command. The rotational
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kinetic energy is computed as 1/2 I w^2, where I is the inertia tensor
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for the aspherical particle and w is its angular velocity, which is
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computed from its angular momentum.
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IMPORTANT NOTE: For "2d models"_dimension.html, particles are treated
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as ellipsoids, not ellipses, meaning their moments of inertia will be
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the same as in 3d.
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A kinetic energy tensor, stored as a 6-element vector, is also
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calculated by this compute. The formula for the components of the
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tensor is the same as the above formula, except that v^2 and w^2 are
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replaced by vx*vy and wx*wy for the xy component, and the appropriate
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elements of the inertia tensor are used. The 6 components of the
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vector are ordered xx, yy, zz, xy, xz, yz.
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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 {dynamic} option of the
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"compute_modify"_compute_modify.html command if this is not the case.
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If a {bias-ID} is specified it must be the ID of a temperature compute
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that removes a "bias" velocity from each atom. This allows compute
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temp/sphere to compute its thermal temperature after the translational
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kinetic energy components have been altered in a prescribed way,
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e.g. to remove a velocity profile. Thermostats that use this compute
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will work with this bias term. See the doc pages for individual
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computes that calculate a temperature and the doc pages for fixes that
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perform thermostatting for more details.
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This compute subtracts out translational degrees-of-freedom due to
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fixes that constrain molecular motion, such as "fix
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shake"_fix_shake.html and "fix rigid"_fix_rigid.html. This means the
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temperature of groups of atoms that include these constraints will be
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computed correctly. If needed, the subtracted degrees-of-freedom can
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be altered using the {extra} option of the
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"compute_modify"_compute_modify.html command.
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See "this howto section"_Section_howto.html#4_16 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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[Output info:]
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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 "this
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section"_Section_howto.html#4_15 for an overview of LAMMPS output
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options.
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The scalar value calculated by this compute is "intensive". The
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vector values are "extensive".
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The scalar value will be in temperature "units"_units.html. The
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vector values will be in energy "units"_units.html.
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[Restrictions:]
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This compute requires that particles be represented as extended
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ellipsoids and not point particles. This means they will have an
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angular momentum and a shape which is determined by the
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"shape"_shape.html command.
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[Related commands:]
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"compute temp"_compute_temp.html
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[Default:] none
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