# [Ifeffit] Question about using a proper model

Matt Newville newville at cars.uchicago.edu
Mon Oct 27 20:29:09 CDT 2014

Pushkar,

On Mon, Oct 27, 2014 at 7:57 PM, pushkar shejwalkar
<pshejwalkar2004 at gmail.com> wrote:
> Dear Matt,
>           Thank you very much for your reply. Yes it generates list of XYZ
> cordinates. I am wondering if its ok to use such information. The reason is
> that while converting the XYZ file into .inp file, the format is different
> and hence I am simply typing the coordinates into it, however I am not sure
> as to should I change nleg values, Rmax values and other mphase mfeff values
> and all. Is it same for all the program? just changing the coordinates is
> enough?

Please understand that we only know as much about what you are trying
to as you actually tell us.
While we can give you advice, we cannot actually solve your problems for you.

I think that what you are trying to do is create a feff.inp file from
atomic coordinates generated by some other program called Spartan
(coincidentally, Greek-themed!).   A feff.inp has several parts to it,
one being a list of X, Y, Z, IPOT, where X, Y, Z are atomic
coordinated (to be clear -- in Angstroms) and IPOT is an integer index
of one of the Potentials listed elsewhere in the file.  There must be
between 2 and 7 IPOTs, number sequentially starting with 0.  There is
exactly 1 IPOT '0' -- this is the absorbing atom.  Each scattering
element must have its own IPOT (including any other elements that are
the same Z as the absorber).

Starting with an existing feff.inp file and modifying the list of
atomic coordinates and, if necessary, the list of potentials,  is
highly recommended.   It's not that hard.

> About lower E0 value, I read in Shelly's paper that mostly ppl do not report E0 value

People do report E0.

> and if they and if it is >10 then there may be problem with
> modelling or so.

An E0 shift > 10 (or < -10) is a warning that something may not be
right.  It's not necessarily wrong, but should cause you to pause and
try to understand such a large discrepancy between what Feff thought
E0 should be and what the maximum of the first derivative is.

> one of the method to reduce such chances of high E0 is by
> properly aligning spectra in athena. She mention in this a method to use
> first artemis for getting first shell fitting using E0=0 and then coming
> back to the athena and using this file as reference. But I am not sure how
> to open artemis file in athena because if I open .fpj file in athena it
> doesnt recognise and says that the format is not readable if you continue
> then athena shutdown. In artemis I cannot save file as .prj. is there a way
> around? OR IT IS JUST THAT I AM DOING IT ALL WRONG?

Just change E0 in Athena, perhaps copying a Group first, and use that
chi(k) with the updated E0 as the data to fit.    Usually Feff wants
E0 further up the edge than the maximum of the first derivative, but
not always.

--Matt