# 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](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

``` LAMMPS
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](pair_eff) 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](fix_nh) 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](fix_nh) doc page), are
performed with computes that include the eFF contribution to the
temperature or kinetic energy from the electron radial velocity.

:::: note
::: title
Note
:::

there are two different pressures that can be reported for eFF when
defining the pair_style (see [pair eff/cut](pair_eff) 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).
::::

:::: note
::: title
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](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 page for the [fix nvt, npt, and nph](fix_nh) commands for
details.

## Restrictions

This fix is part of the EFF package. It is only enabled if LAMMPS was
built with that package. See the [Build package](Build_package) page for
more info.

Other restriction discussed on the page for the [fix nvt, npt, and
nph](fix_nh) commands also apply.

:::: note
::: title
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_nh), [fix nph](fix_nh), [fix npt](fix_nh),
[fix_modify](fix_modify), [run_style](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.

------------------------------------------------------------------------

::: {#Martyna1}
**(Martyna)** Martyna, Tobias and Klein, J Chem Phys, 101, 4177 (1994).
:::

::: {#Parrinello}
**(Parrinello)** Parrinello and Rahman, J Appl Phys, 52, 7182 (1981).
:::

::: {#Tuckerman1}
**(Tuckerman)** Tuckerman, Alejandre, Lopez-Rendon, Jochim, and Martyna,
J Phys A: Math Gen, 39, 5629 (2006).
:::

::: {#Shinoda2}
**(Shinoda)** Shinoda, Shiga, and Mikami, Phys Rev B, 69, 134103 (2004).
:::