Xlens for Powder Data

The powder diffraction part of XLENS is activated with the control parameter IPOW. This keyword is showed in the Intensity Data Dialog in the GXLENS program.



When IPOW=1 the input file with the intensity data is not code.HKL but code.POWThe internal structure (code.POW) is described in the next section. The general strategy when working with powder data is slightly different as for the SC mode due to the difficulty to preset the minimum R value (RVMIN).


In the PW mode it is better to run first all the NSET trials (e.g. 50 or 100), then to analise the different solutions for the one with the lowest R-value and finally develop this solution with an additional XLENS run. To be sure that all NSET trials are computed in the first run, RVMIN must be set very low, e.g. RVMIN = 1.


The Powder Data has also consequences on the control parameters, either by changing the default values:


ELIM

= [1.15]


ERATIO

= [1.05]



DSFOU


= [minimum d-spacing used]

The data resolution limit of the S-FFT parts is controlled with the unique keyword DSFOU (i.e. DSDM is no longer active).

NITER

= [1]

In the PW mode NITER is fixed to 1, i.e. the set of phases refined with S-FFT are directly used to find the listed atomic positions.

EXFAC

= [1.0]

EXFAC · NCELL is the number of atoms sought in the last peak search. In SC, NCELL is the number of non-H atoms in the unit CELL (see CONTENT instruction). In the PW mode NCELL can be modified by the user as explained below.

NSET

= [50]




or by introducing new keywords.


When working with powder data, atomic resolution is seldom reached. In such cases the GRID parameter can be slightly increased (<0.33 * DSFOU) to speed up the calculations.


Keywords


IRODM

ROCUTL

ROCUTH

NCELL

IPICUP


All previous parameters are showed in the S-FFT Dialog when the user select the Powder data option in the Intensity dailog.


Copyright © 2010, Jordi Rius & FullProf Suite Team