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### fix wall/reflect command

Syntax:

    fix ID group-ID wall/reflect keyword ...

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

Examples:

    fix xwalls all wall/reflect xlo xhi
    fix walls all wall/reflect xlo ylo zlo xhi yhi zhi

Description:

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

Reflection means that if an atom moves outside the box on a timestep by a distance delta (e.g. due to fix nve), then it is put back inside the box 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.

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 scalar or vector 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).