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Re: Spin-dependent calculations and DOS resolution
Hi Dave,
We have been doing some spin polarized calculations recently for
clusters with a couple thousand atoms and, to tell you the truth, I
haven't noticed that broadening effect you talk about. In fact, if I'm
correct, you control the broadening for each point of the LDOS with the
eimag parameter. In your case that is set to 0.1 eV, so you can expect
to have very little detail in your LDOS. The problem with the fine
structure of the LDOS, according to what John explained to me once, is
that it has to be consistent with the finite size of your cluster and
that is related to the electron mean free path, if I remember
correctly. You can decrease that parameter, but you need to go to
larger clusters so the LDOS still makes sense. What we did for our
calculations is to try several calculations for clusters of different
sizes and different values of eimag ranging from 0.01 eV to 0.15 eV. We
took the value of eimag for which the LDOS is the same for all the
different clusters. I know that there is a formula (a very simple
expression) to estimate this value beforehand (I think it is simply
that the electron mean free path has to be much smaller than the size
of your cluster, or something like that), but I never seem to find it
when I need it. Maybe John could shed some ligth here :-)
Also, to perform the calculations I was talking about before, we've
used the parallel MPI FEFF. It works magnificently. I've noticed
though, that when we use a lot of processors (128 or 256), sometimes
FEFF crashes. However, I'm not sure if this is a bug in FEFF or a
problem with RMS, which is the queuing system used in this machine.
Also, the MPI libraries used in this cluster are not the standard
MPICH, but some proprietary libs by Compaq, so that could also be part
of the problem. I don't even know if FEFF has been ever tested with so
many processes and for such a demand for memory (like 50 Gb or
something like that for each calculation). In general, though, it is
very stable, it compiles without any problem, and runs very fast so,
definitely, it's a worthy investment.
Hope this helps,
Angel
On Thursday, February 13, 2003, at 08:38 AM, David Eustace wrote:
Hi all,
I have just started doing spin dependent LDOS calculations on CrN, a
simple rock salt structure. I get good correlation with published
data, but a noticeable loss of fine structure compared with non-spin
dependent calculations of the same size. I have already established
the parameters which give the level of fine structure i'm after with
the non-spin dependent case, and I'm simply repeating the calculation
with the SPIN card added (repeating twice, of course, with SPIN 2 then
SPIN -2). The "resolution" of the plot decreases.
Is this a natural manifestation of the SPIN card?
Is it simply a case of making the calculation bigger to get better
resolution?
I should add I'm only interested in obtaining the spin up and spin
down LDOS and XANES.
I include an example of one of my feff.inp files. Please point out if
i'm being too simplistic in my approach to these calculations (I have
a suspicion there must be more to it than this).
Also, I had sent out a question in an earlier email about the modular
feff 8.2 version which can run in parallel, and i didn't get any
response. The manual says that its still 'experimental'. Is there
serious problems with it or does it currently run reasonably reliably?
We're interested in obtaining the modular version but only if it is
non-troublesome!! . Any comments at all would be appreciated.
Thanks for any response,
Dave Eustace <feff.inp>