compute temp command — LAMMPS documentation

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# compute temp command

Accelerator Variants: temp/kk

## Syntax

    compute ID group-ID temp

- ID, group-ID are documented in compute command

- temp = style name of this compute command

## Examples

    compute 1 all temp
    compute myTemp mobile temp

## Description

Define a computation that calculates the temperature of a group of atoms. A compute of this style can be used by any command that computes a temperature, e.g. thermo_modify, fix temp/rescale, fix npt, etc.

The temperature is calculated by the formula

\[T = \frac{2 E_\mathrm{kin}}{N_\mathrm{DOF} k_B} \quad \mathrm{with} \quad E_\mathrm{kin} = \sum^{N_\mathrm{atoms}}_{i=1} \frac{1}{2} m_i v^2_i \quad \mathrm{and} \quad N_\mathrm{DOF} = n_\mathrm{dim} N_\mathrm{atoms} - n_\mathrm{dim} - N_\mathrm{fix DOFs}\]

where \(E_\mathrm{kin}\) is the total kinetic energy of the group of atoms, \(n_\mathrm{dim}\) is the dimensionality of the simulation (i.e. either 2 or 3), \(N_\mathrm{atoms}\) is the number of atoms in the group, \(N_\mathrm{fix DOFs}\) is the number of degrees of freedom removed by fix commands (see below), \(k_B\) is the Boltzmann constant, and \(T\) is the resulting computed temperature.

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.

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. By default this extra component is initialized to \(n_\mathrm{dim}\) (as shown in the formula above) to represent the degrees of freedom removed from a system due to its translation invariance due to periodic boundary conditions.

A compute of this style with the ID of “thermo_temp” is created when LAMMPS starts up, as if this command were in the input script:

    compute thermo_temp all temp

See the “thermo_style” command for more details.

See the Howto thermostat page for a discussion of different ways to compute temperature and perform thermostatting.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

## Output info

This compute calculates a global scalar (the temperature) and a global vector of length six (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

## Related commands

compute temp/partial, compute temp/region, compute pressure

## Default

none