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:

LJ reduced units:

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
input damping   t_freq_in       inverse fmsec
internal temp   t_freq          inverse clicks

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
input press     p_start_in      atmospheres
output press    log file        atmospheres
input damping   p_freq_in       inverse time
internal press  p_freq          inverse clicks

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