compute ke/eff command — LAMMPS documentation

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# compute ke/eff command

## Syntax

    compute ID group-ID ke/eff

- ID, group-ID are documented in compute command

- ke/eff = style name of this compute command

## Examples

    compute 1 all ke/eff

## Description

Define a computation that calculates the kinetic energy of motion of a group of eFF particles (nuclei and electrons), as modeled with the electronic force field.

The kinetic energy for each nucleus is computed as \(\frac{1}{2} m v^2\) and the kinetic energy for each electron is computed as \(\frac{1}{2}(m_e v^2 + \frac{3}{4} m_e s^2)\), where \(m\) corresponds to the nuclear mass, \(m_e\) to the electron mass, \(v\) to the translational velocity of each particle, and \(s\) to the radial velocity of the electron, respectively.

There is a subtle difference between the quantity calculated by this compute and the kinetic energy calculated by the ke or etotal keyword used in thermodynamic output, as specified by the thermo_style command. For this compute, kinetic energy is “translational” and “radial” (only for electrons) kinetic energy, calculated by the simple formula above. For thermodynamic output, the ke keyword infers kinetic energy from the temperature of the system with \(\frac{1}{2} k_B T\) of energy for each degree of freedom. For the eFF temperature computation via the compute temp_eff command, these are the same. But different computes that calculate temperature can subtract out different non-thermal components of velocity and/or include other degrees of freedom.

Warning

The temperature in eFF models should be monitored via the compute temp/eff command, which can be printed with thermodynamic output by using the 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

See compute temp/eff.

## Output info

This compute calculates a global scalar (the KE). This value can be used by any command that uses a global scalar value 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 “extensive”. The scalar value will be in energy units.

## 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

none

## Default

none