Computing Methods in Crystallography by J. S. Rollett

By J. S. Rollett

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ROLLETT L6, §5-6 5. If the refinement involves the positional and anisotropie vibrational parameters of / atoms, plus the scale-factor, we have 9/+1 — q and for / = 20 we have q = 181. This gives q(q+\)/2 = 16,471, and a computer with a large store is necessary. Even for the largest computers available the term in q2 leads to difficulty for relatively small q and it may become necessary to economise space (and computing time) by leaving out some of the off-diagonal terms ofATA which are known to be small.

2. The |F 0 | are usually on an arbitrary scale so that the quantity to be compared with |F C | is ^ | ^ 0 | , where Ä"is a scale-factor and |F 0 | is the observed amplitude provided. The value of K is usually required to be a parameter of the refinement. It is convenient to print out K\F0\ and |JFC|, because this provides quantities which are readily compared with those for other structures. We should not however minimise Σ^Ο^Ι^οΙ — |^c|)2> because the absolute uncertainty of K\F0\ varies with K and we cannot properly assign a fixed weight to it.

ROLLETT [3, §9-11 r The calculation of (<5x . <5x) is relatively slow, and this subroutine is therefore entered only when all other simpler checks pass. 10. The distance routine can be applied to the calculation of angles between short distances in a very straightforward way. Similar concepts are useful in the enumeration of those points of a triclinic lattice lying within a sphere. DIFFRACTOMETER CALCULATIONS 11. Consider a three-circle goniometer, or diffractometer, of the kind shown in Fig. 4.

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