LAUE Instruction

LAUE E

Wavelength-dependent values of f' and f" may be defined for an element E by means of the LAUE instruction, which is used in conjunction with the HKLF 2 reflection data format (in which the wavelength is given separately for each reflection). This is primarily intended for refinement of structures against Laue data collected using synchrotron radiation, but could also be used for refinement of a structure using data collected at different wavelengths employing other sources. There is no provision for handling overlapping reflection orders. Scaling for the source intensity distribution and Lp, absorption corrections etc. must have been performed before using SHELXL_93. A dummy wavelength of say 0.7 Angstrom should be given on the CELL instruction, and the absorption coefficient estimated by the program should be ignored.

The element symbol may be preceded by '$' but this is optional; it must be followed by at least one blank or the end of the line. Any remaining information on the LAUE instruction line is ignored. The line immediately following the LAUE instruction is always ignored, and so may be used for headings. The following lines contain values of wavelength (in Angstroms), f' and f" in FORMAT(F7.3,2F8.3); further information (e.g. mu) may follow on the same line but will be ignored. The wavelength values must be in ascending order and will be linearly interpolated; the wavelength intervals do not need to be equal (but it is more efficient if most of them are) and should indeed be smaller in the region of an absorption edge. This list is terminated by a record in which all three values are given as zero. There should only be one LAUE instruction for each element type; if a reflection wavelength is outside the range specified, the constant f' and f" values defined by the corresponding SFAC instruction are used instead.

A LAUE instruction must be preceded by (normal) SFAC and UNIT instructions referencing the elements in question, and by all atoms. Thus the LAUE instruction(s) are usually the last instructions before HKLF 2 (or -2) at the end of the '.ins' file (which facilitates editing). The +filename construction may conveniently be used to read long LAUE tables from 'include' files without echoing them. If computer memory is restricted (e.g. in the real-mode PC version of SHELXL_93) then the LAUE tables should not cover a larger range than is strictly necessary for the reflection data employed.