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 to
apply 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 Cu
OVERLAP 0 *determine overlap for central atom of Fe

  *iphovr novr rovr * ipot, number in shell, distance
   1	  4    2.15
OVERLAP 1 *determine approximate overlap for 1st shell N
  *iphovr novr rovr * ipot, number in shell, distance
   0	  1    2.15

*SS index ipot deg rss
SS  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=OVERLAP
I think the problem is that you have not specified the SS keyword:

  http://feff.phys.washington.edu/feff/wiki/index.php?title=SS

I use the overlap/ss thing in feff very infrequently.  My preference

is to start from a known structure that is somehow similar to the
material I am investigating.  For example, if I were to look at
amorphous germanium, I would use crystalline germanium as my starting
point.  I would then construct the model in such a way that I

parameterize the differences between the starting structure and what I
expect to find in the data.

I'd recommend reading papers by Scott Calvin or Anatoly Frenkel on
nanoparticles or by Shelly Kelly on uranium in biogeochemical systems

for excellent examples of how that sort of analysis is done.

Hope that helps,
B