# compute temp/ramp command

## Syntax

``` LAMMPS
compute ID group-ID temp/ramp vdim vlo vhi dim clo chi keyword value ...
```

-   ID, group-ID are documented in [compute](compute) command
-   temp/ramp = style name of this compute command
-   vdim = *vx* or *vy* or *vz*
-   vlo,vhi = subtract velocities between vlo and vhi (velocity units)
-   dim = *x* or *y* or *z*
-   clo,chi = lower and upper bound of domain to subtract from (distance
    units)
-   zero or more keyword/value pairs may be appended
-   keyword = *units*

<!-- -->

    *units* value = *lattice* or *box*

## Examples

``` LAMMPS
compute 2nd middle temp/ramp vx 0 8 y 2 12 units lattice
```

## Description

Define a computation that calculates the temperature of a group of
atoms, after subtracting out an ramped velocity profile before computing
the kinetic energy. A compute of this style can be used by any command
that computes a temperature (e.g. [thermo_modify](thermo_modify), [fix
temp/rescale](fix_temp_rescale), [fix npt](fix_nh)).

The meaning of the arguments for this command which define the velocity
ramp are the same as for the [velocity ramp](velocity) command which was
presumably used to impose the velocity.

After the ramp velocity has been subtracted from the specified dimension
for each atom, the temperature is calculated by the formula

$$\text{KE} = \frac{\text{dim}}{2} N k_B T,$$

where KE is the total kinetic energy of the group of atoms (sum of
$\frac12 m v^2$), dim = 2 or 3 is the dimensionality of the simulation,
$N$ is the number of atoms in the group, $k_B$ is the Boltzmann
constant, and $T$ is the absolute temperature.

The *units* keyword determines the meaning of the distance units used
for coordinates (*clo*, *chi*) and velocities (*vlo*, *vhi*). A *box*
value selects standard distance units as defined by the [units](units)
command (e.g., $\AA$ for units = real or metal). A *lattice* value means
the distance units are in lattice spacings (i.e., velocity in lattice
spacings per unit time). The [lattice](lattice) command must have been
previously used to define the lattice spacing.

A kinetic energy tensor, stored as a six-element vector, is also
calculated by this compute for use in the computation of a pressure
tensor. The formula for the components of the tensor is the same as the
above formula, except that $v^2$ is replaced by $v_x v_y$ for the $xy$
component, and so on. The six components of the vector are ordered $xx$,
$yy$, $zz$, $xy$, $xz$, $yz$.

The number of atoms contributing to the temperature is assumed to be
constant for the duration of the run; use the *dynamic* option of the
[compute_modify](compute_modify) command if this is not the case.

The removal of the ramped velocity component by this fix is essentially
computing the temperature after a \"bias\" has been removed from the
velocity of the atoms. If this compute is used with a fix command that
performs thermostatting then this bias will be subtracted from each
atom, thermostatting of the remaining thermal velocity will be
performed, and the bias will be added back in. Thermostatting fixes that
work in this way include [fix nvt](fix_nh), [fix
temp/rescale](fix_temp_rescale), [fix
temp/berendsen](fix_temp_berendsen), and [fix langevin](fix_langevin).

This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as [fix shake](fix_shake) and [fix
rigid](fix_rigid). This means the temperature of groups of atoms that
include these constraints will be computed correctly. If needed, the
subtracted degrees-of-freedom can be altered using the *extra* option of
the [compute_modify](compute_modify) command.

See the [Howto thermostat](Howto_thermostat) page for a discussion of
different ways to compute temperature and perform thermostatting.

## Output info

This compute calculates a global scalar (the temperature) and a global
vector of length 6 (KE tensor), which can be accessed by indices 1\--6.
These values can be used by any command that uses global scalar or
vector values from a compute as input. See the [Howto
output](Howto_output) page for an overview of LAMMPS output options.

The scalar value calculated by this compute is \"intensive\". The vector
values are \"extensive\".

The scalar value will be in temperature [units](units). The vector
values will be in energy [units](units).

## Restrictions

> none

## Related commands

[compute temp](compute_temp), [compute
temp/profie](compute_temp_profile), [compute
temp/deform](compute_temp_deform), [compute pressure](compute_pressure)

## Default

The option default is units = lattice.