forked from lijiext/lammps
250 lines
11 KiB
Plaintext
250 lines
11 KiB
Plaintext
"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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fix rigid :h3
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[Syntax:]
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fix ID group-ID rigid bodystyle args keyword values ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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rigid = style name of this fix command :l
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bodystyle = {single} or {molecule} or {group} :l
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{single} args = none
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{molecule} args = none
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{group} args = N groupID1 groupID2 ...
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N = # of groups
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groupID1, groupID2, ... = list of N group IDs :pre
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zero or more keyword/value pairs may be appended :l
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keyword = {force} or {torque} :l
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{force} values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass force is active
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{torque} values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass torque is active :pre
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:ule
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[Examples:]
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fix 1 clump rigid single
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fix 1 clump rigid single force 1 off off on
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fix 1 polychains rigid molecule
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fix 1 polychains rigid molecule force 1*5 off off off force 6*10 off off on
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fix 2 fluid rigid group 3 clump1 clump2 clump3
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fix 2 fluid rigid group 3 clump1 clump2 clump3 torque * off off off :pre
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[Description:]
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Treat one or more sets of atoms as independent rigid bodies. This
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means that each timestep the total force and torque on each rigid body
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is computed as the sum of the forces and torques on its constituent
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particles and the coordinates, velocities, and orientations of the
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atoms in each body are updated so that the body moves and rotates as a
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single entity.
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Examples of large rigid bodies are a large colloidal particle, or
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portions of a large biomolecule such as a protein.
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Example of small rigid bodies are patchy nanoparticles, such as those
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modeled in "this paper"_#Zhang by Sharon Glotzer's group, clumps of
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granular particles, lipid molecules consiting of one or more point
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dipoles connected to other spheroids or ellipsoids, and coarse-grain
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models of nano or colloidal particles consisting of a small number of
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constituent particles. Note that the "fix shake"_fix_shake.html
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command can also be used to rigidify small molecules of 2, 3, or 4
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atoms, e.g. water molecules. That fix treats the constituent atoms as
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point masses.
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The constituent particles within a rigid body can be point particles
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(the default in LAMMPS) or finite-size particles, such as spheroids
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and ellipsoids. See the "shape"_shape.html command and "atom_style
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granular"_atom_style.html for more details on these kinds of
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particles. Finite-size particles contribute differently to the moment
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of inertia of a rigid body than do point particles. Finite-size
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particles can also experience torque (e.g. due to "frictional granular
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interactions"_pair_gran.html) and have an orientation. These
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contributions are accounted for by the fix.
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Forces between particles within a body do not contribute to the
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external force or torque on the body. Thus for computational
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efficiency, you may wish to turn off pairwise and bond interactions
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between particles within each rigid body. The "neigh_modify
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exclude"_neigh_modify.html and "delete_bonds"_delete_bonds.html
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commands are used to do this. For finite-size particles this also
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means the particles can be highly overlapped when creating the rigid
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body.
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IMPORTANT NOTE: This fix is overkill if you simply want to hold a
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collection of atoms stationary or have them move with a constant
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velocity. A simpler way to hold atoms stationary is to not include
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those atoms in your time integration fix. E.g. use "fix 1 mobile nve"
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instead of "fix 1 all nve", where "mobile" is the group of atoms that
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you want to move. You can move atoms with a constant velocity by
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assigning them an initial velocity (via the "velocity"_velocity.html
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command), setting the force on them to 0.0 (via the "fix
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setforce"_fix_setforce.html command), and integrating them as usual
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(e.g. via the "fix nve"_fix_nve.html command).
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IMPORTANT NOTE: This fix updates the positions and velocities of the
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rigid atoms with a constant-energy time integration, so you should not
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update the same atoms via other fixes (e.g. nve, nvt, npt).
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:line
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Each body must have two or more atoms. An atom can belong to at most
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one rigid body. Which atoms are in which bodies can be defined via
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several options.
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For bodystyle {single} the entire fix group of atoms is treated as one
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rigid body.
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For bodystyle {molecule}, each set of atoms in the fix group with a
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different molecule ID is treated as a rigid body.
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For bodystyle {group}, each of the listed groups is treated as a
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separate rigid body. Only atoms that are also in the fix group are
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included in each rigid body.
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By default, each rigid body is acted on by other atoms which induce an
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external force and torque on its center of mass, causing it to
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translate and rotate. Components of the external center-of-mass force
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and torque can be turned off by the {force} and {torque} keywords.
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This may be useful if you wish a body to rotate but not translate, or
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vice versa, or if you wish it to rotate or translate continuously
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unaffected by interactions with other particles. Note that if you
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expect a rigid body not to move or rotate by using these keywords, you
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must insure its initial center-of-mass translational or angular
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velocity is 0.0. Otherwise the initial translational or angular
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momentum the body has will persist.
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An xflag, yflag, or zflag set to {off} means turn off the component of
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force of torque in that dimension. A setting of {on} means turn on
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the component, which is the default. Which rigid body(s) the settings
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apply to is determined by the first argument of the {force} and
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{torque} keywords. It can be an integer M from 1 to Nbody, where
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Nbody is the number of rigid bodies defined. A wild-card asterisk can
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be used in place of, or in conjunction with, the M argument to set the
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flags for multiple rigid bodies. This takes the form "*" or "*n" or
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"n*" or "m*n". If N = the number of rigid bodies, then an asterisk
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with no numeric values means all bodies from 1 to N. A leading
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asterisk means all bodies from 1 to n (inclusive). A trailing
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asterisk means all bodies from n to N (inclusive). A middle asterisk
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means all types from m to n (inclusive). Note that you can use the
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{force} or {torque} keywords as many times as you like. If a
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particular rigid body has its component flags set multiple times, the
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settings from the final keyword are used.
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For computational efficiency, you may wish to turn off pairwise and
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bond interactions within each rigid body, as they no longer contribute
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to the motion. The "neigh_modify exclude"_neigh_modify.html and
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"delete_bonds"_delete_bonds.html commands are used to do this.
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For computational efficiency, you should define one fix rigid which
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includes all the desired rigid bodies. LAMMPS will allow multiple
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rigid fixes to be defined, but it is more expensive.
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This fix uses constant-energy NVE-style integration, so you may need
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to impose additional constraints to control the temperature of an
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ensemble of rigid bodies. You can use "fix
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langevin"_fix_langevin.html for this purpose to treat the system as
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effectively immersed in an implicit solvent, e.g. a Brownian dynamics
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model. Or you can thermostat only the non-rigid atoms that surround
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one or more rigid bodies (i.e. explicit solvent) by appropriate choice
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of groups in the compute and fix commands for temperature and
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thermostatting.
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If you calculate a temperature for particles in the rigid bodies, the
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degrees-of-freedom removed by each rigid body are accounted for in the
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temperature (and pressure) computation, but only if the temperature
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group includes all the particles in a particular rigid body.
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A 3d rigid body has 6 degrees of freedom (3 translational, 3
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rotational), except for a collection of point particles lying on a
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straight line, which has only 5, e.g a dimer. A 2d rigid body has 3
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degrees of freedom (2 translational, 1 rotational).
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IMPORTANT NOTE: You may wish to explicitly subtract additional
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degrees-of-freedom if you use the {force} and {torque} keywords to
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eliminate certain motions of one or more rigid bodies, as LAMMPS does
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not do this automatically.
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The rigid body contribution to the pressure of the system (virial) is
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also accounted for by this fix.
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IMPORTANT NOTE: The periodic image flags of atoms in rigid bodies are
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modified when the center-of-mass of the rigid body moves across a
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periodic boundary. They are not incremented/decremented as they would
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be for non-rigid atoms. This change does not affect dynamics, but
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means that any diagnostic computation based on the atomic image flag
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values must be adjusted accordingly. For example, the "fix
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msd"_fix_msd.html will not compute the expected mean-squared
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displacement for such atoms, and the image flag values written to a
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"dump file"_dump.html will be different than they would be if the
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atoms were not in a rigid body. It also means that if you have bonds
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between a pair of rigid bodies and the bond straddles a periodic
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boundary, you cannot use the "replicate"_replicate command to increase
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the system size.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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This fix computes a global array of values which can be accessed by
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various "output commands"_Section_howto.html#4_15. The number of rows
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in the array is equal to the number of rigid bodies. The number of
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columns is 12. Thus for each rigid body, 12 values are stored: the
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xyz coords of the center of mass (COM), the xyz components of the COM
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velocity, the xyz components of the force acting on the COM, and the
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xyz components of the torque acting on the COM. The force and torque
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values in the array are not affected by the {force} and {torque}
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keywords in the fix rigid command; they reflect values before any
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changes are made by those keywords.
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The ordering of the rigid bodies (by row in the array) is as follows.
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For the {single} keyword there is just one rigid body. For the
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{molecule} keyword, the bodies are ordered by ascending molecule ID.
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For the {group} keyword, the list of group IDs determines the ordering
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of bodies.
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The array values calculated by this fix are "intensive", meaning they
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are independent of the number of atoms in the simulation.
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:]
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This fix performs an MPI_Allreduce each timestep that is proportional
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in length to the number of rigid bodies. Hence it will not scale well
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in parallel if large numbers of rigid bodies are simulated.
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If the atoms in a single rigid body initially straddle a periodic
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boundary, the input data file must define the image flags for each
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atom correctly, so that LAMMPS can "unwrap" the atoms into a valid
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rigid body.
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[Related commands:]
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"delete_bonds"_delete_bonds.html, "neigh_modify"_neigh_modify.html
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exclude
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[Default:]
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The option defaults are force * on on on and torque * on on on meaning
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all rigid bodies are acted on by center-of-mass force and torque.
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:line
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:link(Zhang)
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[(Zhang)] Zhang, Glotzer, Nanoletters, 4, 1407-1413 (2004).
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