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### fix nvt/eff command

### fix npt/eff command

### fix nph/eff command

Syntax:

    fix ID group-ID style_name keyword value ...

- ID, group-ID are documented in fix command
- style_name = nvt/eff or npt/eff or nph/eff

      one or more keyword value pairs may be appended
      keyword = temp or iso or aniso or tri or x or y or z or xy or yz or xz or couple or tchain or pchain or mtk or tloop or ploop or nreset or drag or dilate
        temp values = Tstart Tstop Tdamp
          Tstart,Tstop = external temperature at start/end of run
          Tdamp = temperature damping parameter (time units)
        iso or aniso or tri values = Pstart Pstop Pdamp
          Pstart,Pstop = scalar external pressure at start/end of run (pressure units)
          Pdamp = pressure damping parameter (time units)
        x or y or z or xy or yz or xz values = Pstart Pstop Pdamp
          Pstart,Pstop = external stress tensor component at start/end of run (pressure units)
          Pdamp = stress damping parameter (time units)
        couple = none or xyz or xy or yz or xz
        tchain value = length of thermostat chain (1 = single thermostat)
        pchain values = length of thermostat chain on barostat (0 = no thermostat)
        mtk value = yes or no = add in MTK adjustment term or not
        tloop value = number of sub-cycles to perform on thermostat
        ploop value = number of sub-cycles to perform on barostat thermostat
        nreset value = reset reference cell every this many timesteps
        drag value = drag factor added to barostat/thermostat (0.0 = no drag)
        dilate value = all or partial

Examples:

    fix 1 all nvt/eff temp 300.0 300.0 0.1
    fix 1 part npt/eff temp 300.0 300.0 0.1 iso 0.0 0.0 1.0
    fix 2 part npt/eff temp 300.0 300.0 0.1 tri 5.0 5.0 1.0
    fix 2 ice nph/eff x 1.0 1.0 0.5 y 2.0 2.0 0.5 z 3.0 3.0 0.5 yz 0.1 0.1 0.5 xz 0.2 0.2 0.5 xy 0.3 0.3 0.5 nreset 1000

Description:

These commands perform time integration on Nose-Hoover style non-Hamiltonian equations of motion for nuclei and electrons in the group for the electron force field model. The fixes are designed to generate positions and velocities sampled from the canonical (nvt), isothermal-isobaric (npt), and isenthalpic (nph) ensembles. This is achieved by adding some dynamic variables which are coupled to the particle velocities (thermostatting) and simulation domain dimensions (barostatting). In addition to basic thermostatting and barostatting, these fixes can also create a chain of thermostats coupled to the particle thermostat, and another chain of thermostats coupled to the barostat variables. The barostat can be coupled to the overall box volume, or to individual dimensions, including the xy, xz and yz tilt dimensions. The external pressure of the barostat can be specified as either a scalar pressure (isobaric ensemble) or as components of a symmetric stress tensor (constant stress ensemble). When used correctly, the time-averaged temperature and stress tensor of the particles will match the target values specified by Tstart/Tstop and Pstart/Pstop.

The operation of these fixes is exactly like that described by the fix nvt, npt, and nph commands, except that the radius and radial velocity of electrons are also updated. Likewise the temperature and pressure calculated by the fix, using the computes it creates (as discussed in the fix nvt, npt, and nph doc page), are performed with computes that include the eFF contribution to the temperature or kinetic energy from the electron radial velocity.

IMPORTANT NOTE: there are two different pressures that can be reported for eFF when defining the pair_style (see pair eff/cut to understand these settings), one (default) that considers electrons do not contribute radial virial components (i.e. electrons treated as incompressible 'rigid' spheres) and one that does. The radial electronic contributions to the virials are only tallied if the flexible pressure option is set, and this will affect both global and per-atom quantities. In principle, the true pressure of a system is somewhere in between the rigid and the flexible eFF pressures, but, for most cases, the difference between these two pressures will not be significant over long-term averaged runs (i.e. even though the energy partitioning changes, the total energy remains similar).

IMPORTANT NOTE: currently, there is no available option for the user to set or create temperature distributions that include the radial electronic degrees of freedom with the velocity command, so the the user must allow for these degrees of freedom to equilibrate (i.e. equi-partitioning of energy) through time integration.

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

See the doc page for the fix nvt, npt, and nph commands for details.

Restrictions:

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

Other restriction discussed on the doc page for the fix nvt, npt, and nph commands also apply.

IMPORTANT NOTE: The temperature for systems (regions or groups) with only electrons and no nuclei is 0.0 (i.e. not defined) in the current temperature calculations, a practical example would be a uniform electron gas or a very hot plasma, where electrons remain delocalized from the nuclei. This is because, even though electron virials are included in the temperature calculation, these are averaged over the nuclear degrees of freedom only. In such cases a corrective term must be added to the pressure to get the correct kinetic contribution.

Related commands:

fix nvt, fix nph, fix npt, fix_modify, run_style

Default:

The keyword defaults are tchain = 3, pchain = 3, mtk = yes, tloop = ploop = 1, nreset = 0, drag = 0.0, dilate = all, and couple = none.

(Martyna) Martyna, Tobias and Klein, J Chem Phys, 101, 4177 (1994).

(Parrinello) Parrinello and Rahman, J Appl Phys, 52, 7182 (1981).

(Tuckerman) Tuckerman, Alejandre, Lopez-Rendon, Jochim, and Martyna, J Phys A: Math Gen, 39, 5629 (2006).

(Shinoda) Shinoda, Shiga, and Mikami, Phys Rev B, 69, 134103 (2004).