diff -Naur lammps-2Jul09/doc/fix_rigid.html lammps-3Jul09/doc/fix_rigid.html --- lammps-2Jul09/doc/fix_rigid.html 2009-05-19 08:43:06.000000000 -0600 +++ lammps-3Jul09/doc/fix_rigid.html 2009-06-30 11:19:42.000000000 -0600 @@ -51,21 +51,52 @@
Description:
-Treat one or more sets of atoms as an independent rigid body. This +
Treat one or more sets of atoms as independent rigid bodies. This means that each timestep the total force and torque on each rigid body -is computed and the coordinates and velocities of the atoms in each -body are updated so that they move as a rigid body. This can be -useful for freezing one or more portions of a large biomolecule, or -for simulating a system of colloidal particles. -
-IMPORTANT NOTE: This fix is overkill if you just want to hold group of -atoms stationary of have them move with a constant velocity. A -simpler way to hold atoms stationary is to not include those atoms in -your time integration fix. E.g. use "fix 1 mobile nve" instead of -"fix 1 all nve", where "mobile" is the group of atoms that you want to -move. You can move atoms with a constant velocity by assigning them -an initial velocity (via the velocity command), -setting the force on them to 0.0 (via the fix +is computed as the sum of the forces and torques on its constituent +particles and the coordinates, velocities, and orientations of the +atoms in each body are updated so that the body moves and rotates as a +single entity. +
+Examples of large rigid bodies are a large colloidal particle, or +portions of a large biomolecule such as a protein. +
+Example of small rigid bodies are patchy nanoparticles, such as those +modeled by the Glotzer group, clumps of granular particles, +lipid molecules consiting of one or more point dipoles connected to +other spheroids or ellipsoids, and coarse-grain models of nano or +colloidal particles consisting of a small number of constituent +particles. Note that the fix shake command can also be +used to rigidify small molecules of 2, 3, or 4 atoms, e.g. water +molecules. That fix treats the constituent atoms as point masses. +
+The constituent particles within a rigid body can be point particles +(the default in LAMMPS) or finite-size particles, such as spheroids +and ellipsoids. See the shape command and atom_style +granular for more details on these kinds of +particles. Finite-size particles contribute differently to the moment +of inertia of a rigid body than do point particles. Finite-size +particles can also experience torque (e.g. due to frictional granular +interactions) and have an orientation. These +contributions are accounted for by the fix. +
+Forces between particles within a body do not contribute to the +external force or torque on the body. Thus for computational +efficiency, you may wish to turn off pairwise and bond interactions +between particles within each rigid body. The neigh_modify +exclude and delete_bonds +commands are used to do this. For finite-size particles this also +means the particles can be highly overlapped when creating the rigid +body. +
+IMPORTANT NOTE: This fix is overkill if you simply want to hold a +collection of atoms stationary or have them move with a constant +velocity. A simpler way to hold atoms stationary is to not include +those atoms in your time integration fix. E.g. use "fix 1 mobile nve" +instead of "fix 1 all nve", where "mobile" is the group of atoms that +you want to move. You can move atoms with a constant velocity by +assigning them an initial velocity (via the velocity +command), setting the force on them to 0.0 (via the fix setforce command), and integrating them as usual (e.g. via the fix nve command).
@@ -73,6 +104,8 @@ rigid atoms with a constant-energy time integration, so you should not update the same atoms via other fixes (e.g. nve, nvt, npt). +Each body must have two or more atoms. An atom can belong to at most one rigid body. Which atoms are in which bodies can be defined via several options. @@ -214,4 +247,10 @@
The option defaults are force * on on on and torque * on on on meaning all rigid bodies are acted on by center-of-mass force and torque.
+(Zhang) Zhang, Glotzer, Nanoletters, 4, 1407-1413 (2004). +