[Wien] atomic form factor calculation

[HW Sheng]

Dr. Gorfman,

Thanks very much. Now I have in mind a better picture as to how to solve
this problem. Your suggestions are very valuable.

Best,

Howard

>-----Original Message-----
>From: wien-bounces at zeus.theochem.tuwien.ac.at
>[mailto:wien-bounces at zeus.theochem.tuwien.ac.at]On Behalf Of Semen
>Gorfman
>Sent: Wednesday, April 26, 2006 8:45 AM
>To: A Mailing list for WIEN2k users
>Subject: Re[2]: [Wien] atomic form factor calculation
>
>
>Dear Howard Sheng!
>
>Generally speaking there are no any unambiguous ways to subdivide
>electron density in solids into independent atomic fragments /
>pseudoatom. In other words there are no individual atoms in crystal
>unit cell.
>
>For X-ray structure analysis several different approximation are
>used. Bader(zero flux surfaces) atomic partitioning is one possible
>way, but there are also some other ones. For example: multipole model
>or Hishfelld approaches. Finally, free atoms model may be used.
>Depending on the type of pseudoatoms (Bader volumes, multipole or
>Hirshfeld pseudoatoms) you get different Fourier transformation of
>them, and accordingly different form factors of a model pseudoatom in
>a crystal.
>
>Hirshfeld approach might be technically the simplest one. First you
>get the weight function, dividing electron density in a crystal by
>electron density in pro-crystal (superposition of free atomic
>densities). Then you just multiply this weight function with the free
>atomic density of an atom.
>
>Best regards
>Semen Gorfman
>
>
>> Dr. Marks,
>
>> Thanks for the paper. I have read it and it's an excellent paper.
>
>> I understand that the chemical bonding introduces changes to the atomic
>> factor(s), due to charge redistribution and orbital
>hybridization. It's not
>> surprising that dstart generates the same atomic factor as the tabulated
>> values, since dstart basically superposes the charge densities
>of all atoms,
>> without relaxing to their electronical ground states.
>
>> My problem still remains: how can I generate the atomic form factor of
>> individual atoms in the unit cell? In my simulation of the X-ray
>structure
>> factor (e.g. of a liquid), I need not only <f>^2, but also
><f^2>. It's the
>> latter that I found difficult to get.
>
>> I come by a solution, but am not sure if it will work. One may
>do the Bader
>> analysis first, and the charge density can be divided into regions of
>> different atoms. The subsequent Fourier transform of the charge
>density (of
>> different atoms) then gives the atomic form factors. How do you
>think? Are
>> there other simpler workarounds?
>
>> Thank you,
>
>> Howard
>
>>>-----Original Message-----
>>>From: wien-bounces at zeus.theochem.tuwien.ac.at
>>>[mailto:wien-bounces at zeus.theochem.tuwien.ac.at]On Behalf Of L. D. Marks
>>>Sent: Tuesday, April 25, 2006 10:18 PM
>>>To: A Mailing list for WIEN2k users
>>>Subject: Re: [Wien] atomic form factor calculation
>>>
>>>
>>>See http://scripts.iucr.org/cgi-bin/paper?lc5043 . The tabulated x-ray
>>>scattering factors and those from dstart are essentially the same, the
>>>changes on bonding are much larger.
>>>
>>>On Tue, 25 Apr 2006, HW Sheng wrote:
>>>
>>>> Dear Wien users,
>>>>
>>>> I wonder if I can use lapw3 to calculate the atomic form factor for
>>>> individual atoms in an alloy.
>>>>
>>>> If I understand this correctly, lapw3 computes the atomic form factor
>>>> (so-called X-ray structure factor) of a unit cell, including
>>>contributions
>>>> from all elements (if not an elemental solid) in the unit cell.
>>>The is the
>>>> averaged atomic form factor <f>. Any guidance/suggestions how to
>>>calculate
>>>> the form factor of individual atoms?  I need this in order to
>compare how
>>>> the atomic form factor in an alloy ( or a compound) differs from the
>>>> tabulated value of  "neutral atoms", and this information will
>help me to
>>>> simulate X-ray diffraction experiments with high accuracy.
>>>>
>>>> Your help would be greatly appreciated.
>>>>
>>>> Howard Sheng
>>>>
>>>> Dept. Materials Science & Engineering
>>>> Johns Hopkins University
>>>>
>>>>
>>>
>>>Note: if you have an old email address for me, please note that "nwu" has
>>>been changed to "northwestern".
>>>-----------------------------------------------
>>>Laurence Marks
>>>Department of Materials Science and Engineering
>>>MSE Rm 2036 Cook Hall
>>>2225 N Campus Drive
>>>Northwestern University
>>>Evanston, IL 60208, USA
>>>Tel: (847) 491-3996 Fax: (847) 491-7820
>>>email: L-marks at northwestern dot edu
>>>http://www.numis.northwestern.edu
>>>-----------------------------------------------
>>>
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>>>http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
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