\(\renewcommand{\AA}{\text{Å}}\)

# compute temp/ramp command

## Syntax

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

ID, group-ID are documented in 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*

unitsvalue =latticeorbox

## Examples

```
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, fix temp/rescale, fix npt).

The meaning of the arguments for this command which define the velocity ramp are the same as for the velocity ramp 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

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
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
command must have been previously used to define the lattice spacing.

A symmetric tensor, stored as a six-element vector, is also calculated by this compute for use in the computation of a pressure tensor by the compute pressue command. The formula for the components of the tensor is the same as the above expression for \(E_\mathrm{kin}\), except that the 1/2 factor is NOT included and the \(v_i^2\) is replaced by \(v_{i,x} v_{i,y}\) for the \(xy\) component, and so on. Note that because it lacks the 1/2 factor, these tensor components are twice those of the traditional kinetic energy tensor. 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 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 temp/rescale, fix temp/berendsen, and fix langevin.

This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as fix shake and
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 command.

See the 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 (symmetric 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 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 is in temperature units. The vector values are in energy units.

## Restrictions

none

## Default

The option default is units = lattice.