fix command

Syntax:

fix ID style args 

• ID = user-assigned name for the fix
• style = one of a long list of possible style names (see below)
• args = arguments used by a particular style

Examples:

fix 1 grid/check 100 warn
fix 1 ave/time all 100 5 1000 c_myTemp c_thermo_temp file temp.profile 

Description:

Set a fix that will be applied to the system. In SPARTA, a "fix" is an operation that is applied to the system during timestepping. Examples include adding particles via inlet boundary conditions or computing diagnostics. Code for new fixes can be added to SPARTA; see Section 10 of the manual for details.

Fixes perform their operations at different stages of the timestep. If 2 or more fixes operate at the same stage of the timestep, they are invoked in the order they were specified in the input script.

The ID for a fix is used to identify the fix in other commands. Each fix ID must be unique; see an exception below. The ID can only contain alphanumeric characters and underscores. You can specify multiple fixes of the same style so long as they have different IDs. A fix can be deleted with the unfix command, after which its ID can be re-used.

IMPORTANT NOTE: The unfix command is the only way to turn off a fix; simply specifying a new fix with the same style and a different ID will not turn off the first one.

If you specify a new fix with the same ID and style as an existing fix, the old fix is deleted and the new one is created (presumably with new settings). This is the same as if an "unfix" command were first performed on the old fix, except that the new fix is kept in the same order relative to the existing fixes as the old one originally was.

Some fixes store an internal "state" which is written to binary restart files via the restart or write_restart commands. This allows the fix to continue on with its calculations 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. See the doc pages for individual fixes for info on which ones can be restarted.

Each fix style has its own doc page which describes its arguments and what it does, as listed below. Here is an alphabetic list of fix styles available in SPARTA:

• ablate - alter implicit surfaces within each grid cell
• adapt - on-the-fly grid adaptation
• ambipolar - ambipolar approximation for ionized plasmas
• ave/grid - compute per grid cell time-averaged quantities
• ave/histo - compute/output time averaged histograms
• ave/histo/weight - compute/output weighted histograms
• ave/surf - compute per surface element time-averaged quantities
• ave/time - compute/output global time-averaged quantities
• balance - perform dynamic load-balancing
• dt/reset - adjust global timestep dynamically
• emit/face - emit particles at global boundaries
• emit/face/file - emit particles at global boundaries using a distribution defined in a file
• emit/surf - emit particles at surfaces
• field/grid - apply an external field on a per grid cell basis
• field/particle - apply an external field on a per particle basis
• grid/check - check if particles are in the correct grid cell
• move/surf - move surfaces dynamically during a simulation
• print - print text and variables during a simulation
• surf/temp - compute per-surf temperatures dynamically
• temp/global/rescale - rescale particle temperatures
• temp/rescale - rescale particle temperatures within each grid cell
• surf/temp - compute per-surf temperatures dynamically
• vibmode - discrete vibrational energy modes

There are also additional accelerated compute styles included in the SPARTA distribution for faster performance on specific hardware. The list of these with links to the individual styles are given in the pair section of this page.

In addition to the operation they perform, some fixes also produce one of four styles of quantities: global, per-particle, per-grid, or per-surf. These can be used by other commands or output as described below. A global quantity is one or more system-wide values, e.g. the temperature of the system. A per-particle quantity is one or more values per particle, e.g. the kinetic energy of each particle. A per-grid quantity is one or more values per grid cell. A per-surf quantity is one or more values per surface element.

Global, per-particle, per-grid, and per-surf quantities each come in two forms: a single scalar value or a vector of values. Additionaly, global quantities can also be a 2d array of values. The doc page for each fix describes the style and kind of values it produces, e.g. a per-particle vector. Some fixes can produce more than one form of a single style, e.g. a global scalar and a global vector.

When a fix quantity is accessed, as in many of the output commands discussed below, it can be referenced via the following bracket notation, where ID is the ID of the fix:

In other words, using one bracket reduces the dimension of the quantity once (vector -> scalar, array -> vector). Using two brackets reduces the dimension twice (array -> scalar). Thus a command that uses scalar fix values as input can also process elements of a vector or array.

Note that commands and variables which use fix quantities typically do not allow for all kinds, e.g. a command may require a vector of values, not a scalar. This means there is no ambiguity about referring to a fix quantity as f_ID even if it produces, for example, both a scalar and vector. The doc pages for various commands explain the details.

Any values generated by a fix can be used in several ways:

• Global values can be output via the stats_style command. Or the values can be referenced in a variable equal or variable atom command.
• Per-particle values can be output via the dump particle command. Or the per-particle values can be referenced in an particle-style variable.
• Per-grid values can be output via the dump grid command. Or the per-grid values can be referenced in a grid-style variable.

Restrictions: none

Related commands:

unfix

Default: none