SFAC elementsElement symbols which define the order of scattering factors to be employed by the program. The first 94 elements of the periodic system are recognized. The element name may be preceded by '$' but this is not obligatory (the '$' character is allowed for logical consistency but is ignored). The program uses the neutral atom scattering factors, f' and f" and absorption coefficients from International Tables for Crystallography, Volume C (1992), Ed. A.J.C. Wilson, Kluwer Academic Publishers, Dordrecht: Tables 188.8.131.52 (pp. 500-502), 184.108.40.206 (pp. 219-222) and 220.127.116.11 (pp. 193-199) respectively. The covalent radii stored in the program are based on experience rather than taken from a specific source, and are deliberately overestimated for elements which tend to have variable coordination numbers so that 'bonds' are not missed, at the cost of generating the occasional 'non-bond'. The default radii (not those set for individual atoms by CONN) are printed before the connectivity table.
SFAC label a1 b1 a2 b2 a3 b3 a4 b4 c f' f" mu r wtScattering factor in the form of an exponential series, followed by real and imaginary dispersion terms, linear absorption coefficient, covalent radius and atomic weight. Except for the 'label' and atomic weight the format is the same as that used in SHELX-76. label consists of up to 4 characters beginning with a letter (e.g. Ca2+) and should be included before a1; the first label character may be a '$', but this would be ignored (note however that the '$', if used, counts as one of the four characters). The two SFAC formats may be used in the same '.ins' file; the order of the SFAC instructions (and the order of element names in the first type of SFAC instruction) define the scattering factor numbers which are referenced by atom instructions. The units of mu should be barns/atom, as in Table 18.104.22.168 of International Tables, Volume C (see above).
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