RE: Combined neutron/x-ray refinements

[P . G . Radaelli]

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