SIMWGT will make up to three passes through the reflection record on the bdf. In the first pass the scale factors between and are calculated. The overall scale factor and rescale factors for the sinθ/λ bins are computed and displayed as scale factors on F, not. If no rescaling is selected on the SIMWGT line, this first pass is skipped. On the second pass, the scale factors are used to calculate <|-|> for the bins and on the third pass, the phase probabilities, "figures of merit", are calculated, stored on the bdf, and statistics listed.

The phase probability for phases generated from a set of Fc's, whether produced from a model, density modification, etc, is based on the work of Sim (1960) who expressed the phase probablity as

Hendrickson and Lattman recast this into the form of

and

where

In Sim's original formulation, Σ_L represents the contribution to intensities of all the atoms not modelled ( modelled incorrectly) in the accumulation of Fc. Bricogne has suggested obtaining Σ_L empirically from the discrepancy between observed and calculated intensities. This is accomplished by computing in small resolution shells.

This phase information can be combined with MIR phase profiles (fazprb option) based on a formulation for the phase probablitiy distributions (Hendrickson and Lattman, 1970), (Hendrickson, 1971). Arbitrary probability distributions can be formulated in terms of 4 trigonometric coefficients (A,B,C,D).

This form has the advantage that phase probabilities may be combined by simple addition of the constant coefficients. The coefficients can be derived from an analysis of the sources of phase error in the MIR determination. The combined phase can then be obtained by integrating the probablities around the phase circle of

Reads reflection record from input archive bdf

Writes output archive bdf

SIMWGT

In this example, Sim weights will be used to calculate figure of merits from the Fo in item 1304 and the Fc in item 1800 from the lrrefl: on the bdf. Default sinθ/λ bins will be used. Scale factors will be calculated for the individual bins and used in calculating the figure of merits. The results will be put out in items 705 (FOM), 706 (cosα), and 707 (sinα).

SIMWGT 100 0

scale 14.35

maxhkl *4 0.005 0.25

slimit 2.00

In this example, scale factors will NOT be calculated for the individual bins and the input scale factor will be used in calculating the figures of merit. The sinθ/λ limits on the maxhkl line and a 2σ cutoff will be used to restrict the data processed. The results will be put out in items 705 (FOM), 706 (cosα), and 707 (sinα). The first 100 reflection information will be listed.

Bricogne, G. (1976) Acta Cryst. A32, 832.

Hendrickson, W.A., E.E. Lattman (1970) Acta Cryst., B26, 136-143.

Hendrickson, W.A. (1971) Acta Cryst., B27, 1472-1473.

Sim, G.A. (1960) Acta Cryst., 13, 511.