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[转载]lammps学习必看6

已有 1580 次阅读 2021-8-15 21:49 |个人分类:lammps|系统分类:科研笔记|文章来源:转载

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.

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

来源:https://docs.lammps.org/99/units.html



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