As chi^2 is a function of the number of data points included in the
refinement, combined refinements have considerably improved values for a
total chi^2 when compared with refinements carried out against individual
data sets.
Correspondingly the ESDs in the combined refinement output should be
significantly lower than those obtained from a single data set refinement
unless there is something drastically wrong with the application of
combined refinement to the particular problem (e.g. preferred orientation,
surface vs bulk etc).
It is my experience that the combined refinement chi^2 is always lower
than that obtained from using just (say) the neutron data. We have
frequently collected data sets at both room temperature and 5 K using D2b.
The room temperature data are refined simultaneously with lab X-ray data
to give a chi^2 of 2.02 whilst the D2b data collected at 5 K refined as a
single data set gives chi^2 of 4.53 (published in JACS, 1999, 121,
3958-3967). In my experience this improvement in chi^2 is typical.
Eddie Cussen
Inorganic Chemistry Laboratory,
Department of Chemistry,
University of Oxford,
South Parks Road,
Oxford, OX1 3QR
United Kingdom
E-mail: [EMAIL PROTECTED]
tel: (..44)(0)1865-272602
Fax: (..44)(0)1865-272690
On Tue, 25 May 1999 [EMAIL PROTECTED] wrote:
> Jon Wright wrote:
>
> >I guess the degradation which is found would come from parameters which
> >are determined by both datasets and come out with different values in each
> >separate refinement.
>
> Not necessarily. In order to get the ESD, the variance-covariance matrix is
> multiplied by chi^2, and the roots of the diagonal elements are taken.
> Therefore, if the chi^2 of the combined refinement is worse than that of the
> individual ones, the ESD will automatically be worsened. I think this is by
> far the commonest case. Also, by adding reflections that are insensitive to
> a given parameter my feeling is that you increase the esd on that parameter
> even if chi^2=1, but the proof of this is too tedious.
>
> Paolo
>
