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fix nvt command

Syntax:

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

Examples:

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

Description:

Perform constant NVT integration to update positions and velocities each timestep for atoms 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 run command documents how to make the ramping take place across multiple runs. 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", as if this command had been issued:

compute fix-ID_temp group-ID temp 

See the >compute temp command for details. Note that the ID of the new compute is the fix-ID with 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. Alternatively, you can directly assign a new compute (for calculating temeperature) that you have defined to this fix via the fix_modify command.

This fix makes a contribution to the potential energy of the system that can be included in thermodynamic output of potential energy using the fix_modify energy option. The contribution can also be printed by itself via the keyword f_fix-ID in the thermo_style custom command.

Restrictions:

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 nve, fix npt, fix temp/rescale, fix langevin, fix_modify, temperature

Default:

The keyword defaults are drag = 0.0.