SPARTA WWW Site - SPARTA Documentation - SPARTA Commands

fix temp/rescale command

fix temp/rescale/kk command


fix ID temp/rescale N Tstart Tstop keyword value ... 


fix 1 temp/rescale 100 300.0 300.0
fix 5 temp/rescale 10 300.0 10.0 ave yes 


Reset the thermal temperature of all the particles within each grid cell by explicitly rescaling their thermal velocities. This is a simple thermostatting operation to keep the thermal temperature of the gas near the desired target temperature. This can be useful if an external driving force is adding energy to the system. Or if you wish the thermal temperature of the system to heat or cool over time.

The rescaling is applied to only the translational degrees of freedom for the particles. Their rotational or vibrational degrees of freedom are not altered.

Rescaling is performed every N timesteps. The target temperature (Ttarget) is a ramped value between the Tstart and Tstop temperatures at the beginning and end of the run.

This fix performs thermostatting on a per grid cell basis. For each grid cell, the center-of-mass velocity and thermal temperature of the particles in the grid cell is computed. The thermal temperature is defined as the kinetic temperature after any center-of-mass motion (e.g. a flow velocity) is subtracted from the collection of particles in the grid cell. These are the same calculations as are performed by the compute thermal/grid command. See its doc page for the equations. See the fix temp/global/rescale doc page for a command that thermostats the temperature of the global system.

How the rescaling of particle velocities is done depends on the value of the ave keyword.

For ave with a value no (the default), the thermal temperature (Tthermal) of each cell is used to compute a velocity scale factor for that cell, which is

vscale = sqrt(Ttarget/Tthermal) 

The vscale factor is applied to each of the components of the thermal velocity for each particle in the grid cell. Only cells with 2 or more particles have their particle velocities rescaled.

For ave with a value yes, the thermal temperatures of all the cells are averaged. The average thermal temperature is simply the sum of cell thermal temperatures divided by the number of cells. Cells with less than 2 particles or whose thermal temperature = 0.0 contribute a thermal temperaure = Ttarget to the average. The average thermal temperature (Tthermal_ave) for all cells is used to compute a velocity scale factor for all cells, which is

vscale = sqrt(Ttarget/Tthermal_ave) 

This single vscale factor is applied to each of the components of the thermal velocity for each particle in all the grid cells, including the particles in single-particle cells.

After rescaling, for either ave = no or yes, if the thermal temperature were re-computed for any grid cell with more than one particle, it would be exactly the target temperature.

Restart, output info:

No information about this fix is written to binary restart files.

This fix produces no output.

This fix can ramp its target temperature over multiple runs, using the start and stop keywords of the run command. See the run command for details of how to do this.

Styles with a kk suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed in the Accelerating SPARTA section of the manual. The accelerated styles take the same arguments and should produce the same results, except for different random number, round-off and precision issues.

These accelerated styles are part of the KOKKOS package. They are only enabled if SPARTA was built with that package. See the Making SPARTA section for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke SPARTA, or you can use the suffix command in your input script.

See the Accelerating SPARTA section of the manual for more instructions on how to use the accelerated styles effectively.

Restrictions: none

Related commands:

fix temp/global/rescale


The default is ave = no.