compute temp/eff command — LAMMPS documentation

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# compute temp/eff command

## Syntax

    compute ID group-ID temp/eff

- ID, group-ID are documented in compute command

- temp/eff = style name of this compute command

## Examples

    compute 1 all temp/eff
    compute myTemp mobile temp/eff

## Description

Define a computation that calculates the temperature of a group of nuclei and electrons in the electron force field model. A compute of this style can be used by commands that compute a temperature (e.g., thermo_modify, fix npt/eff).

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 \(\frac12 m v^2\) for nuclei and sum of \(\frac12 (m v^2 + \frac34 m s^2\)) for electrons, where \(s\) includes the radial electron velocity contributions), dim = 2 or 3 is the dimensionality of the simulation, \(N\) is the number of atoms (only total number of nuclei in the eFF (see the pair_eff command) in the group, \(k_B\) is the Boltzmann constant, and \(T\) is the absolute temperature. This expression is summed over all nuclear and electronic degrees of freedom, essentially by setting the kinetic contribution to the heat capacity to \(\frac32 k\) (where only nuclei contribute). This subtlety is valid for temperatures well below the Fermi temperature, which for densities two to five times the density of liquid hydrogen ranges from 86,000 to 170,000 K.

Note

For eFF models, in order to override the default temperature reported by LAMMPS in the thermodynamic quantities reported via the thermo command, the user should apply a thermo_modify command, as shown in the following example:

    compute         effTemp all temp/eff
    thermo_style    custom step etotal pe ke temp press
    thermo_modify   temp effTemp

A six-component kinetic energy tensor 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, etc. For the eFF, again, the radial electronic velocities are also considered.

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.

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

## Output info

The scalar value calculated by this compute is “intensive”, meaning it is independent of the number of atoms in the simulation. The vector values are “extensive”, meaning they scale with the number of atoms in the simulation.

## Restrictions

This compute is part of the EFF package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

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

## Default

none