LAMMPS Units

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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