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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 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.

By default the temperature is computed only on the atoms in the fix group using the default temperature style. This can be changed by assigning a different temperature style to the fix via the fix_modify command.

This fix supports the fix_modify options for thermo and energy. The former will print the contribution the fix makes to the energy of the system when thermodynamics is printed. The latter will add this contribution to the total potential energy (PotEng) so that energy conservation can be monitored.

Restrictions: none

Related commands:

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

Default:

The keyword defaults are drag = 0.0.