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### fix wall/reflect command

Syntax:

    fix ID group-ID wall/reflect face arg ... keyword value ...

- ID, group-ID are documented in fix command
- wall/reflect = style name of this fix command
- one or more face/arg pairs may be appended
- face = xlo or xhi or ylo or yhi or zlo or zhi

        xlo,ylo,zlo arg = EDGE or constant or variable
          EDGE = current lo edge of simulation box
          constant = number like 0.0 or -30.0 (distance units)
          variable = equal-style variable like v_x or v_wiggle
        xhi,yhi,zhi arg = EDGE or constant or variable
          EDGE = current hi edge of simulation box
          constant = number like 50.0 or 100.3 (distance units)
          variable = equal-style variable like v_x or v_wiggle

- zero or more keyword/value pairs may be appended
- keyword = units

        units value = lattice or box
          lattice = the wall position is defined in lattice units
          box = the wall position is defined in simulation box units

Examples:

    fix xwalls all wall/reflect xlo EDGE xhi EDGE
    fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
    fix top all wall/reflect zhi v_pressdown

Description:

Bound the simulation with one or more walls which reflect particles in the specified group when they attempt to move thru them.

Reflection means that if an atom moves outside the wall on a timestep by a distance delta (e.g. due to fix nve), then it is put back inside the face by the same delta, and the sign of the corresponding component of its velocity is flipped.

When used in conjunction with fix nve and run_style verlet, the resultant time-integration algorithm is equivalent to the primitive splitting algorithm (PSA) described by Bond. Because each reflection event divides the corresponding timestep asymmetrically, energy conservation is only satisfied to O(dt), rather than to O(dt^2) as it would be for velocity-Verlet integration without reflective walls.

Up to 6 walls or faces can be specified in a single command: xlo, xhi, ylo, yhi, zlo, zhi. A lo face reflects particles that move to a coordinate less than the wall position, back in the hi direction. A hi face reflects particles that move to a coordinate higher than the wall position, back in the lo direction.

The position of each wall can be specified in one of 3 ways: as the EDGE of the simulation box, as a constant value, or as a variable. If EDGE is used, then the corresponding boundary of the current simulation box is used. If a numeric constant is specified then the wall is placed at that position in the appropriate dimension (x, y, or z). In both the EDGE and constant cases, the wall will never move. If the wall position is a variable, it should be specified as v_name, where name is an equal-style variable name. In this case the variable is evaluated each timestep and the result becomes the current position of the reflecting wall. Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent wall position.

The units keyword determines the meaning of the distance units used to define a wall position, but only when a numeric constant or variable is used. It is not relevant when EDGE is used to specify a face position. In the variable case, the variable is assumed to produce a value compatible with the units setting you specify.

A box value selects standard distance units as defined by the units command, e.g. Angstroms for units = real or metal. A lattice value means the distance units are in lattice spacings. The lattice command must have been previously used to define the lattice spacings.

Here are examples of variable definitions that move the wall position in a time-dependent fashion using equal-style variables.

    variable ramp equal ramp(0,10)
    fix 1 all wall/reflect xlo v_ramp

    variable linear equal vdisplace(0,20)
    fix 1 all wall/reflect xlo v_linear

    variable wiggle equal swiggle(0.0,5.0,3.0)
    fix 1 all wall/reflect xlo v_wiggle

    variable wiggle equal cwiggle(0.0,5.0,3.0)
    fix 1 all wall/reflect xlo v_wiggle

The ramp(lo,hi) function adjusts the wall position linearly from lo to hi over the course of a run. The vdisplace(c0,velocity) function does something similar using the equation position = c0 + velocity*delta, where delta is the elapsed time.

The swiggle(c0,A,period) function causes the wall position to oscillate sinusoidally according to this equation, where omega = 2 PI / period:

    position = c0 + A sin(omega*delta)

The cwiggle(c0,A,period) function causes the wall position to oscillate sinusoidally according to this equation, which will have an initial wall velocity of 0.0, and thus may impose a gentler perturbation on the particles:

    position = c0 + A (1 - cos(omega*delta))

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. 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:

Any dimension (xyz) that has a reflecting wall must be non-periodic.

A reflecting wall should not be used with rigid bodies such as those defined by a "fix rigid" command. This is because the wall/reflect displaces atoms directly rather than exerts a force on them. For rigid bodies, use a soft wall instead, such as fix wall/lj93. LAMMPS will flag the use of a rigid fix with fix wall/reflect with a warning, but will not generate an error.

Related commands:

fix wall/lj93 command

Default: none

(Bond) Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).