forked from lijiext/lammps
120 lines
3.4 KiB
HTML
120 lines
3.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<HTML>
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<HEAD>
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<META NAME="Generator" CONTENT="Cosmo Create 1.0.3">
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</HEAD>
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<BODY>
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<H2>
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LAMMPS Units</H2>
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<P>
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<A HREF="README.html">Return</A> to top-level LAMMPS documentation.</P>
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<P>
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This file describes the units associated with many of the key variables
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and equations used inside the LAMMPS code. Units used for input command
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parameters are described in the input_commands file. The input command
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"units" selects between conventional and Lennard-Jones units.
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See the force_fields file for more information on units for the force
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field parameters that are input from data files or input scripts. </P>
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<P>
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Conventional units: </P>
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<UL>
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<LI>
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distance = Angstroms
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<LI>
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time = femtoseconds
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<LI>
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mass = grams/mole
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<LI>
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temperature = degrees K
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<LI>
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pressure = atmospheres
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<LI>
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energy = Kcal/mole
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<LI>
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velocity = Angstroms/femtosecond
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<LI>
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force = grams/mole * Angstroms/femtosecond^2
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<LI>
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charge = +/- 1.0 is proton/electron
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</UL>
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<P>
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LJ reduced units: </P>
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<UL>
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<LI>
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distance = sigmas
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<LI>
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time = reduced LJ tau
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<LI>
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mass = ratio to unitless 1.0
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<LI>
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temperature = reduced LJ temp
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<LI>
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pressure = reduced LJ pressure
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<LI>
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energy = epsilons
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<LI>
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velocity = sigmas/tau
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<LI>
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force = reduced LJ force (sigmas/tau^2)
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<LI>
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charge = ratio to unitless 1.0
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</UL>
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<HR>
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<P>
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This listing of variables assumes conventional units; to convert to LJ
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reduced units, simply substitute the appropriate term from the list
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above. E.g. x is in sigmas in LJ units. Per-mole in any of the units
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simply means for 6.023 x 10^23 atoms.</P>
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<P>
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</P>
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<PRE>
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Meaning Variable Units
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positions x Angstroms
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velocities v Angstroms / click (see below)
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forces f Kcal / (mole - Angstrom)
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masses mass gram / mole
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charges q electron units (-1 for an electron)
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(1 e.u. = 1.602 x 10^-19 coul)
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time --- clicks (1 click = 48.88821 fmsec)
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timestep dt clicks
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input timestep dt_in fmsec
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time convert dtfactor 48.88821 fmsec / click
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temperature t_current degrees K
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t_start
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t_stop
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input damping t_freq_in inverse fmsec
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internal temp t_freq inverse clicks
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damping
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dielec const dielectric 1.0 (unitless)
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Boltmann const boltz 0.001987191 Kcal / (mole - degree K)
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virial virial[xyz] Kcal/mole = r dot F
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pressure factor pfactor 68589.796 (convert internal to atmospheres)
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internal p_current Kcal / (mole - Angs^3)
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pressure p_start
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p_stop
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input press p_start_in atmospheres
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p_stop_in
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output press log file atmospheres
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input damping p_freq_in inverse time
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internal press p_freq inverse clicks
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damping
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pot eng e_potential Kcal/mole
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kin eng e_kinetic Kcal/mole
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eng convert efactor 332.0636 (Kcal - Ang) / (q^2 - mole)
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(convert Coulomb eng to Kcal/mole)
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LJ coeffs lja,ljb Kcal-Angs^(6,12)/mole
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bond various see force_fields file
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parameters 2,3,4-body
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terms
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</PRE>
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</BODY>
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</HTML>
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