esta.measure.atomic_units
¶
In electronic structure calculations, we generally employ “atomic units” (a.u.): here three basic units are the fundamental constants i.e. electronic charge, electron mass, and hbar (reduced Planck's constatn).
Here we do the conversion between atomic units (Hartree) and SI units, as well as conversion between Ryd atomic units and SI units.
Generally atomic units (au) means Hartree units, atomic Rydberg units (aru) means Rydberg units, and natural units means natural units.
Right now conversion between atomic units and Si units supported.
For details see: From: http://physics.nist.gov/constants Also see the pdf file in zot. for details.
.. note::
AU or au: Atomic units:
h_bar = me = e = 1 (The units in the electron's world.)
ARU or aru: Atomic Rydberg Units:
Atomic Rydberg units (ARU) defined by: h_bar = 2me = e^2/2 = 1 (and 4*pi*epsiom = 1)
So the unit for angular momentum is h_bar, the unit for mass is 2m and the unit for charge is e/√2 ... CTM sk!!
#-------------------------------------------------------------- 1 Hartree = 2 Ryd
! mass_e in au_ryd = 2 * mass_e in au-hartree unit, BUT energy in au_ryd = ½ * energy in au-hartree unit
=> mass in au-ryd is double than au-hartree
=> energy in au-ryd is half than in au-hartree #--------------------------------------------------------------
#=============================================================
All Q-E codes use atomic Rydberg units
# i.e. energy in Ry, lengths in Bohr radii
(or \hbar=1, e^2=2, electron mass m=1/2) => charge on e=sqrt(2); mass of e = 1/2 in rau
# with the exception of CP that uses atomic Hartree units
i.e. energy in Ha, lengths in Bohr radii
(or \hbar=1, e=1, electron mass m=1)
#=============================================================
.. tip:: How it works:
This module converts SI units to atomic units i.e. Hartree units (SI to Ryd is on the same line)!!
e.g. how to convert 0.529 angstrom to au units?
In [7]: import esta.measure.atomic_units as atu
In [8]: 0.529e-10 * atu.length2au
Out[8]: 0.9996651248084494
In [8]: atu.ao2au *.529e-10
Out[8]: 0.9996651248084494
e.g. convert 82nN force to au units:
In [16]: 82e-9 * atu.force2au
Out[16]: 0.9953000213791756
author: skumar
email : sonukumar.physics@gmail.com