Some think they don't need "|Fobs|",
are they so sure ?


Absolutely need "|Fobs|"
Don't need, though some use "|Fobs|"
Direct Methods
Patterson
Molecular replacement or
molecule/fragment location
by 
Monte Carlo
Genetic Algorithm
Global Optimization
Packing Considerations (...)

 
 
 
 
 

But at least, the Pawley or Le Bail methods are selected for getting the best starting profile and cell parameters...
 

Using the raw data is time computer consuming : 20000 Monte Carlo moves meaning 20000 powder pattern calculations and checking with the observed data.
 
 
 


 
 
 
 
 
 
 
 

A new process partly overcomes this problem, by using "|Fobs|".

A pseudo pattern is regenerated from the "|Fobs|" with same U,V,W, but without background, Lorentz-polarisation, asymmetry, and with an optimal number of points (3 to 5 steps above the FWHM line).

Calculations on this pseudo pattern are much faster (x3).
 
 
 


 
 
 

Implemented in ESPOIR :
example of the cimetidine solved either from "scratch" or from molecule location

With ESPOIR,
structures may come across by chance,
thanks to Monte Carlo.
 
 
 
 
 
 

The 100 first reflections of cimetidine
in the regenerated powder pattern :


 
 

Compared to the raw pattern at the "|Fobs|" extraction stage :


Demonstration of ESPOIR will be given
at the Software Fayre, 9th August