# fix bocs command

## Syntax

``` LAMMPS
fix ID group-ID bocs keyword values ...
```

-   ID, group-ID are documented in [fix](fix) command

-   bocs = style name of this fix command

-   two or more keyword/value pairs may be appended

-   keyword = *temp* or *cgiso* or *tchain* or *pchain* or *mtk* or
    *tloop* or *ploop*

        *temp* values = Tstart Tstop Tdamp
        *cgiso* values = Pstart Pstop Pdamp basis_set args
          basis_set = *analytic* or *linear_spline* or *cubic_spline*
            *analytic* args = V_avg N_particles N_coeff Coeff_1 Coeff_2 ... Coeff_N
            *linear_spline* args = input_filename
            *cubic_spline* args = input_filename
        *tchain* value = N = length of thermostat chain (1 = single thermostat)
        *pchain* value = N = length of thermostat on barostat (0 = no thermostat)
        *mtk* value = *yes* or *no* = add MTK adjustment term or not
        *tloop* value = M = number of sub-cycles to perform on thermostat
        *ploop* value = M = number of sub-cycles to perform on barostat

## Examples

``` LAMMPS
fix 1 all bocs temp 300.0 300.0 100.0 cgiso 0.986 0.986 1000.0 analytic 66476.015 968 2 245030.10 8962.20
fix 1 all bocs temp 300.0 300.0 100.0 cgiso 0.986 0.986 1000.0 cubic_spline input_Fv.dat
thermo_modify press 1_press
```

## Description

These commands incorporate a pressure correction as described by Dunn
and Noid [(Dunn1)](bocs-Dunn1) to the standard MTK barostat by Martyna
et al. [(Martyna)](bocs-Martyna). The first half of the command mimics a
standard [fix npt](fix_nh) command:

``` LAMMPS
fix 1 all bocs temp Tstart Tstop Tcoupl cgiso Pstart Pstop Pdamp
```

The two differences are replacing *npt* with *bocs*, and replacing
*iso*/\*aniso\*/etc. with *cgiso*. The rest of the command details what
form you would like to use for the pressure correction equation. The
choices are: *analytic*, *linear_spline*, or *cubic_spline*.

With either spline method, the only argument that needs to follow it is
the name of a file that contains the desired pressure correction as a
function of volume. The file must be formatted so each line has:

    Volume_i, PressureCorrection_i

Note both the COMMA and the SPACE separating the volume\'s value and its
corresponding pressure correction. The volumes in the file must be
uniformly spaced. Both the volumes and the pressure corrections should
be provided in the proper units (e.g., if you are using *units real*,
the volumes should all be in $\mathrm{\mathring{A}}^3$ and the pressure
corrections should all be in atm). Furthermore, the table should
start/end at a volume considerably smaller/larger than you expect your
system to sample during the simulation. If the system ever reaches a
volume outside of the range provided, the simulation will stop.

With the *analytic* option, the arguments are as follows:

    ... analytic V_avg N_particles N_coeff Coeff_1 Coeff_2 ... Coeff_N

Note that *V_avg* and *Coeff_i* should all be in the proper units (e.g.,
if you are using *units real*, *V_avg* should be in
$\mathrm{\mathring{A}^3}$ and the coefficients should all be in
$\mathrm{atm}\cdot\mathrm{\mathring{A}^3}$).

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

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

This fix writes the cumulative global energy change to [binary restart
files](restart). See the [read_restart](read_restart) command for info
on how to re-specify a fix in an input script that reads a restart file,
so that the fix continues in an uninterrupted fashion.

The [fix_modify](fix_modify) *temp* option is supported by this fix. You
can use it to assign a temperature [compute](compute) you have defined
to this fix which will be used in its thermostatting procedure, as
described above. For consistency, the group used by this fix and by the
compute should be the same.

The cumulative energy change in the system imposed by this fix is
included in the [thermodynamic output](thermo_style) keywords *ecouple*
and *econserve*. See the [thermo_style](thermo_style) doc page for
details.

This fix computes a global scalar which can be accessed by various
[output commands](Howto_output). The scalar is the same cumulative
energy change due to this fix described in the previous paragraph. The
scalar value calculated by this fix is \"extensive\".

This fix can ramp its target temperature over multiple runs, using the
*start* and *stop* keywords of the [run](run) command. See the
[run](run) command for details of how to do this.

This fix is not invoked during [energy minimization](minimize).

## Restrictions

As this is computing a (modified) pressure, group-ID should be *all*.

The pressure correction has only been tested for use with an isotropic
pressure coupling in 3 dimensions.

By default, LAMMPS will still report the normal value for the pressure
if the pressure is printed via a *thermo* command, or if the pressures
are written to a file every so often. In order to have LAMMPS report the
modified pressure, you must include the *thermo_modify* command given in
the examples. For the last argument in the command, you should put
XXXX_press, where XXXX is the ID given to the fix bocs command (in the
example, the ID of the fix bocs command is 1).

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

## Further information

For more details about the pressure correction and the entire BOCS
software package, visit the [BOCS package on
GitHub](https://github.com/noid-group/BOCS)\_ and read the release paper
by Dunn et al. [(Dunn2)](bocs-Dunn2) .

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

::: {#bocs-Dunn1}
**(Dunn1)** Dunn and Noid, J Chem Phys, 143, 243148 (2015).
:::

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

::: {#bocs-Dunn2}
**(Dunn2)** Dunn, Lebold, DeLyser, Rudzinski, and Noid, J. Phys. Chem.
B, 122, 3363 (2018).
:::