## LAMMPS Units

Return to top-level LAMMPS documentation.

This file describes the units associated with many of the key variables and equations used inside the LAMMPS code. Units used for input command parameters are described in the input_commands file. The input command "units" selects between conventional and Lennard-Jones units. See the force_fields file for more information on units for the force field parameters that are input from data files or input scripts.

Conventional units:

- distance = Angstroms
- time = femtoseconds
- mass = grams/mole
- temperature = degrees K
- pressure = atmospheres
- energy = Kcal/mole
- velocity = Angstroms/femtosecond
- force = grams/mole * Angstroms/femtosecond^2
- charge = +/- 1.0 is proton/electron

LJ reduced units:

- distance = sigmas
- time = reduced LJ tau
- mass = ratio to unitless 1.0
- temperature = reduced LJ temp
- pressure = reduced LJ pressure
- energy = epsilons
- velocity = sigmas/tau
- force = reduced LJ force (sigmas/tau^2)
- charge = ratio to unitless 1.0

This listing of variables assumes conventional units; to convert to LJ reduced units, simply substitute the appropriate term from the list above. E.g. x is in sigmas in LJ units. Per-mole in any of the units simply means for 6.023 x 10^23 atoms.

    Meaning        Variable        Units

    positions      x               Angstroms
    velocities     v               Angstroms / click (see below)
    forces         f               Kcal / (mole - Angstrom)
    masses         mass            gram / mole
    charges        q               electron units (-1 for an electron)
                                     (1 e.u. = 1.602 x 10^-19 coul)

    time           ---             clicks (1 click = 48.88821 fmsec)
    timestep       dt              clicks
    input timestep dt_in           fmsec
    time convert   dtfactor        48.88821 fmsec / click

    temperature     t_current       degrees K
                    t_start
                    t_stop
    input damping   t_freq_in       inverse fmsec
    internal temp   t_freq          inverse clicks
      damping

    dielec const    dielectric      1.0 (unitless)
    Boltmann const  boltz           0.001987191 Kcal / (mole - degree K)

    virial          virial[xyz]     Kcal/mole = r dot F
    pressure factor pfactor         68589.796 (convert internal to atmospheres)
    internal        p_current       Kcal / (mole - Angs^3)
      pressure      p_start
                    p_stop
    input press     p_start_in      atmospheres
                    p_stop_in
    output press    log file        atmospheres
    input damping   p_freq_in       inverse time
    internal press  p_freq          inverse clicks
      damping

    pot eng         e_potential     Kcal/mole
    kin eng         e_kinetic       Kcal/mole
    eng convert     efactor         332.0636 (Kcal - Ang) / (q^2 - mole)
                                    (convert Coulomb eng to Kcal/mole)

    LJ coeffs       lja,ljb         Kcal-Angs^(6,12)/mole

    bond            various         see force_fields file
      parameters    2,3,4-body
                    terms