fix meso/move command — LAMMPS documentation

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# fix meso/move command

## Syntax

    fix ID group-ID meso/move style args keyword values ...

- ID, group-ID are documented in fix command

- meso/move = style name of this fix command

- style = linear or wiggle or rotate or variable

      linear args = Vx Vy Vz
        Vx,Vy,Vz = components of velocity vector (velocity units), any component can be specified as NULL
      wiggle args = Ax Ay Az period
        Ax,Ay,Az = components of amplitude vector (distance units), any component can be specified as NULL
        period = period of oscillation (time units)
      rotate args = Px Py Pz Rx Ry Rz period
        Px,Py,Pz = origin point of axis of rotation (distance units)
        Rx,Ry,Rz = axis of rotation vector
        period = period of rotation (time units)
      variable args = v_dx v_dy v_dz v_vx v_vy v_vz
        v_dx,v_dy,v_dz = 3 variable names that calculate x,y,z displacement as function of time, any component can be specified as NULL
        v_vx,v_vy,v_vz = 3 variable names that calculate x,y,z velocity as function of time, any component can be specified as NULL

- zero or more keyword/value pairs may be appended

- keyword = units

      units value = box or lattice

## Examples

    fix 1 boundary meso/move wiggle 3.0 0.0 0.0 1.0 units box
    fix 2 boundary meso/move rotate 0.0 0.0 0.0 0.0 0.0 1.0 5.0
    fix 2 boundary meso/move variable v_myx v_myy NULL v_VX v_VY NULL

## Description

Perform updates of position, velocity, internal energy and local density for mesoscopic particles in the group each timestep using the specified settings or formulas, without regard to forces on the particles. This can be useful for boundary, solid bodies or other particles, whose movement can influence nearby particles.

The operation of this fix is exactly like that described by the fix move command, except that particles’ density, internal energy and extrapolated velocity are also updated.

Note

The particles affected by this fix should not be time integrated by other fixes (e.g. fix sph, fix sph/stationary), since that will change their positions and velocities twice.

Note

As particles move due to this fix, they will pass through periodic boundaries and be remapped to the other side of the simulation box, just as they would during normal time integration (e.g. via the fix sph command). It is up to you to decide whether periodic boundaries are appropriate with the kind of particle motion you are prescribing with this fix.

Note

As discussed below, particles are moved relative to their initial position at the time the fix is specified. These initial coordinates are stored by the fix in “unwrapped” form, by using the image flags associated with each particle. See the dump custom command for a discussion of “unwrapped” coordinates. See the Atoms section of the read_data command for a discussion of image flags and how they are set for each particle. You can reset the image flags (e.g. to 0) before invoking this fix by using the set image command.

The linear style moves particles at a constant velocity, so that their position X = (x,y,z) as a function of time is given in vector notation as

    X(t) = X0 + V * delta

where X0 = (x0,y0,z0) is their position at the time the fix is specified, V is the specified velocity vector with components (Vx,Vy,Vz), and delta is the time elapsed since the fix was specified. This style also sets the velocity of each particle to V = (Vx,Vy,Vz). If any of the velocity components is specified as NULL, then the position and velocity of that component is time integrated the same as the fix sph command would perform, using the corresponding force component on the particle.

Note that the linear style is identical to using the variable style with an equal-style variable that uses the vdisplace() function. E.g.

    variable V equal 10.0
    variable x equal vdisplace(0.0,$V)
    fix 1 boundary move variable v_x NULL NULL v_V NULL NULL

The wiggle style moves particles in an oscillatory fashion, so that their position X = (x,y,z) as a function of time is given in vector notation as

    X(t) = X0 + A sin(omega*delta)

where X0 = (x0,y0,z0) is their position at the time the fix is specified, A is the specified amplitude vector with components (Ax,Ay,Az), omega is 2 PI / period, and delta is the time elapsed since the fix was specified. This style also sets the velocity of each particle to the time derivative of this expression. If any of the amplitude components is specified as NULL, then the position and velocity of that component is time integrated the same as the fix sph command would perform, using the corresponding force component on the particle.

Note that the wiggle style is identical to using the variable style with equal-style variables that use the swiggle() and cwiggle() functions. E.g.

    variable A equal 10.0
    variable T equal 5.0
    variable omega equal 2.0*PI/$T
    variable x equal swiggle(0.0,$A,$T)
    variable v equal v_omega*($A-cwiggle(0.0,$A,$T))
    fix 1 boundary move variable v_x NULL NULL v_v NULL NULL

The rotate style rotates particles around a rotation axis R = (Rx,Ry,Rz) that goes through a point P = (Px,Py,Pz). The period of the rotation is also specified. The direction of rotation for the particles around the rotation axis is consistent with the right-hand rule: if your right-hand thumb points along R, then your fingers wrap around the axis in the direction of rotation.

This style also sets the velocity of each particle to (omega cross Rperp) where omega is its angular velocity around the rotation axis and Rperp is a perpendicular vector from the rotation axis to the particle.

The variable style allows the position and velocity components of each particle to be set by formulas specified via the variable command. Each of the 6 variables is specified as an argument to the fix as v_name, where name is the variable name that is defined elsewhere in the input script.

Each variable must be of either the equal or atom style. Equal-style variables compute a single numeric quantity, that can be a function of the timestep as well as of other simulation values. Atom-style variables compute a numeric quantity for each particle, that can be a function per-atom quantities, such as the particle’s position, as well as of the timestep and other simulation values. Note that this fix stores the original coordinates of each particle (see note below) so that per-atom quantity can be used in an atom-style variable formula. See the variable command for details.

The first 3 variables (v_dx,v_dy,v_dz) specified for the variable style are used to calculate a displacement from the particle’s original position at the time the fix was specified. The second 3 variables (v_vx,v_vy,v_vz) specified are used to compute a velocity for each particle.

Any of the 6 variables can be specified as NULL. If both the displacement and velocity variables for a particular x,y,z component are specified as NULL, then the position and velocity of that component is time integrated the same as the fix sph command would perform, using the corresponding force component on the particle. If only the velocity variable for a component is specified as NULL, then the displacement variable will be used to set the position of the particle, and its velocity component will not be changed. If only the displacement variable for a component is specified as NULL, then the velocity variable will be used to set the velocity of the particle, and the position of the particle will be time integrated using that velocity.

The units keyword determines the meaning of the distance units used to define the linear velocity and wiggle amplitude and rotate origin. This setting is ignored for the variable style. A box value selects standard units as defined by the units command, e.g. velocity in Angstroms/fs and amplitude and position in Angstroms for units = real. A lattice value means the velocity units are in lattice spacings per time and the amplitude and position are in lattice spacings. The lattice command must have been previously used to define the lattice spacing. Each of these 3 quantities may be dependent on the x,y,z dimension, since the lattice spacings can be different in x,y,z.

## Restart, fix_modify, output, run start/stop, minimize info

This fix writes the original coordinates of moving particles to binary restart files, as well as the initial timestep, so that the motion can be continuous in a restarted simulation. See the read_restart command for info on how to re-specify a fix in an input script that reads a restart file, so that the operation of the fix continues in an uninterrupted fashion.

Note

Because the move positions are a function of the current timestep and the initial timestep, you cannot reset the timestep to a different value after reading a restart file, if you expect a fix move command to work in an uninterrupted fashion.

None of the fix_modify options are relevant to this fix.

This fix produces a per-atom array which can be accessed by various output commands. The number of columns for each atom is 3, and the columns store the original unwrapped x,y,z coords of each particle. The per-atom values can be accessed on any timestep.

No parameter of this fix can be used with the start/stop keywords of the run command.

This fix is not invoked during energy minimization.

## Restrictions

This fix is part of the DPD-SMOOTH package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

This fix requires that atoms store density and internal energy as defined by the atom_style sph command.

All particles in the group must be mesoscopic SPH/SDPD particles.

Changed in version 29Aug2024.

This fix is incompatible with deformation controls that remap velocity, for instance the remap v option of fix deform.

## Related commands

fix move, fix sph, displace_atoms

## Default

The option default is units = lattice.