[Wien] What is the E-cut

[saeid jalali]

The E_cut as it comes from its name is a cut off for
energy to expand the wave functions.
In principle the number of Fourier expansion
coefficients must be infinite, however, in practice it
is impossible to sum numerically many numbers.
Thus numerically one needs to set essentially an
energy cut off to truncate the infinite sum somewhere
that the order of magnitude of the coefficients is
reducing so that adding the truncated coefficients
could not affect the result any more in our desired
accuracy.
For the most cases Fermi energy is less than 1Ry
(around 0.5Ry), so If you would not add spin-orbit
interaction 1.5 Ry could be a good choice for E_cut,
see the end of case.in1 file.
However, in the case of taking spin-orbit (SO)
coupling into account one must increase the E_cut from
1.5Ry to something more or less between 7Ry to 10Ry
considering a suitable convergence on some parameters.
This is due to the fact that adding SO is a
relativistic effect that makes further splitting the
orbitlas. For example (switching on SO) p will be
splitted to p1/2 and p3/2. The wave function of p1/2
like s (and as opposed to the other orbitals with
l<>0) can penetrate into the nuclear. This means that
the value of the wave function of p1/2 in the vicinity
of origin colud not be neglected. The variations of
wave functions near the nuclear are so large that
could not be expanded by a few expansion coefficients,
and one should take a large E_cut to reproduce the
behavior of the wave function correctly as we are
using a full-potential method (and not a
pseudopotential one).


Your,
Saeid. 

--- Chur Allen <anin1996 at hotmail.com> wrote:
> Dear wien users:
>    Recently, I read some papaers about the wien
> calculations. I dont 
> understand the E-cut.
> It is said to be the size of the basis
> functions.There are E-cut for s,p,d 
> and the E-cut for spin orbit.They range from 0 to
> 150 ev.My question is what 
> is E-cut , how to find it after a  calculation,and
> how we make use of it? 
> Can you give me a explanation?
> 
>
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