[Ifeffit] How to use "OVERLAP" card?

[Anatoly Frenkel]

Hi,

Below is an old but working example. See the comment about the units (Bohr's
units in SS card v. Angstrom in Overlap card) - it used to be a bug in
FEFF6 - and it may still be unless Bruce/Matt changed the code in the recent
distribution. If rescaling the distances in your SS card does not help, swap
the SS and the OVERLAP cards (in my example SS preceeds OVERLAP - may be it
matters).

Let me know if it helped,

Anatoly

TITLE   Cu metal in aqueous solution
 TITLE   Structure is from Beagley et al, J. Phys.: Cond. Matt. 1, 2395
(1989)
 TITLE   Distance units are in Angstroms in OVERLAP cards
 TITLE   Distance units are in Angstroms/0.529177249 in SS cards

 HOLE 1   1.0     Cu K edge  (  8.979 keV), s0^2=1.0

 *         mphase,mpath,mfeff,mchi
 CONTROL   1      1     1     1
 PRINT     1      0     0     0


 POTENTIALS
 *   ipot   z  label
       0   29   Cu
       1   8    O
       2   8    O
       3   1    H
       4   8    O

 SS  1  1  4   3.760555
 SS  2  2  2   4.327473
 SS  3  4  11  7.464418

* note that the distances above correspond to the three shells:
*1.99, 2.29 and 3.95 A. However, because of the bug in FEFF6,
*these distances should are used in FEFF6 in the units of Bohr's radius.
*Therefore, for the program to work properly, you should divide the true
*shell distance by a0 = 0.529177249 to enter in SS cards, as in the example
above
*The overlap distances below stay in Angstroms, as they should.


 OVERLAP 0
* Cu - O_eq
  1  4  1.99
* Cu - O_ax
  2  2  2.29
 OVERLAP 1
* O_eq - H
  3  2  0.957
* O_eq - Cu
  0  1  1.99
 OVERLAP 2
* O_ax - H
  3  2  0.957
* O_ax - Cu
  0  1  2.29
 OVERLAP 3
* H - O_eq
  1  1  0.957
* H - H
  3  1  1.513
* H - O_2sh
  4  1  0.957
 OVERLAP 4
* O_2sh - H
  3  2  0.957


  -----Original Message-----
  From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of Jun Lin
  Sent: Tuesday, May 08, 2007 10:07 PM
  To: ifeffit at millenia.cars.aps.anl.gov
  Subject: [Ifeffit] How to use "OVERLAP" card?


I have read some references of Kelly Shelly, and I have got some idea of how
to construct a structure without difinite crystalline structure. But when I
try toapply to my own question, meet some difficulties.
In my case, four nitrogen atoms is surrounding a iron atom. So I use
OVERLAP and SS card as to define such system as described in the FEFF
manual:--------------------------------------------------POTENTIALS
 *    ipot   Z  element                    0   26   Fe
1   7    N                * Determine approximate overlap for central and
1st nn in CuOVERLAP 0 *determine overlap for central atom of Fe
  *iphovr novr rovr * ipot, number in shell, distance   1  	  4
2.15OVERLAP 1 *determine approximate overlap for 1st shell N  *iphovr novr
rovr * ipot, number in shell, distance   0  	  1    2.15
*SS index ipot deg rssSS  1	  1    4
2.15--------------------------------------------------but feff8 stopped and
said:                              Absorbing atom coords not specified.
Cannot find multiple scattering paths.
since the OVERLAP does't need ATOMES card, where can I define the
coordination of absorbing atom?Is there any other CARD needed for such
situation?========================================= Hi Jun,
That keyword is explained here:
http://feff.phys.washington.edu/feff/wiki/index.php?title=OVERLAPI think the
problem is that you have not specified the SS keyword:
  http://feff.phys.washington.edu/feff/wiki/index.php?title=SSI use the
overlap/ss thing in feff very infrequently.  My preference
is to start from a known structure that is somehow similar to thematerial I
am investigating.  For example, if I were to look atamorphous germanium, I
would use crystalline germanium as my startingpoint.  I would then construct
the model in such a way that I
parameterize the differences between the starting structure and what Iexpect
to find in the data.I'd recommend reading papers by Scott Calvin or Anatoly
Frenkel onnanoparticles or by Shelly Kelly on uranium in biogeochemical
systems
for excellent examples of how that sort of analysis is done.Hope that
helps,B

  --
  Shanghai Institute of Applied Physics, Chinese Academy of Sciences
  No. 2019, Baojia Road,
  Jiading District, Shanghai,China 201800
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