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### fix nvt/asphere command

Syntax:

    fix ID group-ID nvt/asphere Tstart Tstop Tdamp keyword value ...

- ID, group-ID are documented in fix command
- nvt/asphere = style name of this fix command
- Tstart,Tstop = desired temperature at start/end of run
- Tdamp = temperature damping parameter (time units)
- zero or more keyword/value pairs may be appended
- keyword = drag

        drag value = drag factor added to thermostat (0.0 = no drag)

Examples:

    fix 1 all nvt/asphere 300.0 300.0 100.0
    fix 1 all nvt/asphere 300.0 300.0 100.0 drag 0.2

Description:

Perform constant NVT integration to update positions, velocities, and angular velocities each timestep for aspherical or ellipsoidal particles in the group using a Nose/Hoover temperature thermostat. V is volume; T is temperature. This creates a system trajectory consistent with the canonical ensemble.

The desired temperature at each timestep is a ramped value during the run from Tstart to Tstop. The Tdamp parameter is specified in time units and determines how rapidly the temperature is relaxed. For example, a value of 100.0 means to relax the temperature in a timespan of (roughly) 100 time units (tau or fmsec or psec - see the units command).

In some cases (e.g. for solids) the temperature of the system can oscillate undesirably when a Nose/Hoover thermostat is applied. The optional drag keyword will damp these oscillations, although it alters the Nose/Hoover equations. A value of 0.0 (no drag) leaves the Nose/Hoover formalism unchanged. A non-zero value adds a drag term; the larger the value specified, the greater the damping effect. Performing a short run and monitoring the temperature is the best way to determine if the drag term is working. Typically a value between 0.2 to 2.0 is sufficient to damp oscillations after a few periods.

This fix computes a temperature each timestep. To do this, the fix creates its own compute of style "temp/asphere", as if this command had been issued:

    compute fix-ID_temp group-ID temp/asphere

See the compute temp/asphere command for details. Note that the ID of the new compute is the fix-ID with underscore + "temp" appended and the group for the new compute is the same as the fix group.

Note that this is NOT the compute used by thermodynamic output (see the thermo_style command) with ID = thermo_temp. This means you can change the attributes of this fix's temperature (e.g. its degrees-of-freedom) via the compute_modify command or print this temperature during thermodyanmic output via the thermo_style custom command using the appropriate compute-ID. It also means that changing attributes of thermo_temp will have no effect on this fix.

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

This fix writes the state of the Nose/Hoover thermostat to binary restart files. 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.

The fix_modify temp option is supported by this fix. You can use it to assign a compute you have defined to this fix which will be used in its thermostatting procedure.

The fix_modify energy option is supported by this fix to add the energy change induced by Nose/Hoover thermostatting to the system's potential energy as part of thermodynamic output.

The potential energy change due to this fix is stored as a scalar quantity, which can be accessed by various output commands.

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.

This fix is not invoked during energy minimization.

Restrictions:

This fix is part of the "asphere" package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

The final Tstop cannot be 0.0 since it would make the target T = 0.0 at some timestep during the simulation which is not allowed in the Nose/Hoover formulation.

Related commands:

fix nvt, fix nve_asphere, fix npt_asphere, fix_modify

Default:

The keyword defaults are drag = 0.0.