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rLAMMPS lammps
units.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<META NAME="Generator" CONTENT="Cosmo Create 1.0.3">
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<H2>
LAMMPS Units</H2>
<P>
<A HREF="README.html">Return</A> to top-level LAMMPS documentation.</P>
<P>
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. </P>
<P>
Conventional units: </P>
<UL>
<LI>
distance = Angstroms
<LI>
time = femtoseconds
<LI>
mass = grams/mole
<LI>
temperature = degrees K
<LI>
pressure = atmospheres
<LI>
energy = Kcal/mole
<LI>
velocity = Angstroms/femtosecond
<LI>
force = grams/mole * Angstroms/femtosecond^2
<LI>
charge = +/- 1.0 is proton/electron
</UL>
<P>
LJ reduced units: </P>
<UL>
<LI>
distance = sigmas
<LI>
time = reduced LJ tau
<LI>
mass = ratio to unitless 1.0
<LI>
temperature = reduced LJ temp
<LI>
pressure = reduced LJ pressure
<LI>
energy = epsilons
<LI>
velocity = sigmas/tau
<LI>
force = reduced LJ force (sigmas/tau^2)
<LI>
charge = ratio to unitless 1.0
</UL>
<HR>
<P>
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.</P>
<P>
</P>
<PRE>
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
</PRE>
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