FVAR osf [1] free variablesThe overall scale factor is followed by the values of the 'free variables' fv(2) ... The overall scale factor is given throughout as the square root of the scale factor which multiplies Fc2 in the least-squares refinement [to make it similar to the scale factor in SHELX-76 which multiplied Fc], i.e. osf2*Fc2 is fitted to Fo2.
SHELXL-93 goes to some trouble to ensure that the initial value of the scale factor has very little influence. Firstly a quick structure factor summation with a small fraction of the total number of reflections is performed to estimate a new scale factor. If the values differ substantially then the new value is used. Secondly the scale factor is factored out of the least-squares algebra so that, although it is still refined, the only influence the previous value has is an indirect one via the weighting scheme and extinction correction.
Before calculating electron density maps and the analysis of variance, and writing the structure factor file ('name.fcf'), the observed F2 values and esd's are brought onto an absolute scale by dividing by the scale factor.
The free variables allow extra constraints to be applied to the atoms, e.g. for common site occupation factors or isotropic displacement parameters, and may be used in conjunction with the SUMP, DFIX and CHIV restraints. If there is more than one FVAR instruction, they are concatenated; they may appear anywhere between UNIT and HKLF (or END).
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