Re: Ab initio
Peter Y. Zavalij ( (no email) )
Thu, 5 Feb 1998 09:18:46 -0500
Actually "ab initio" or whatever it is called does have a place
in solving structure (from powder data or single crystal --
doesn't matter) but only on the first stage when you apply
direct methods. Then (second stage) you use any available
information chemical, crystal-chemical, etc. etc. to make you
choice. In other words to decide which set makes more sense
and how many atoms from this set can be taken to the third
stage -- completing the model. Finally, the last 4th stage --
Rietveld refinement which usually overlaps with previous.
So, only 25% is "ab initio" -- direct methods and, of course,
you could make it more 75% or even more if you refuse
or cannot apply any additional info. Simply paste a few
highest peaks from E, F or D-maps w/o any attempts
to make sense of structure geometry.
Well, you will have to do it any way on the final stages.
As Armel wrote: "That would be almost really "ab initio".
And that would be stupid !:-). "
NEVERTHELESS sometimes it can help if your expectations
are quite different than reality... different composition or
even elements! OR something really new...
Dr. Peter Y. Zavalij University Crystallographer
Materials Research Center, SUNY at Binghamton
Ph/Fax:(607)777-4623 E-mail:zavalij@binghamton.edu
http://imr.chem.binghamton.edu/zavalij/zavalij.html