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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: CoreCIF revision 2.3 ATOM_TYPE_SCAT****From**:**Howard Flack <Howard.Flack@cryst.unige.ch>***Date*:*Fri, 27 Jun 2003 10:39:23 +0100 (BST)**Reply-To*:*coredmg@iucr.org*

> COMMENTS ON #6 > -------------- > ATOM_TYPE_SCAT category > _atom_type_scat_factor_id > _atom_type_scat_factor_type_symbol > _atom_type_scat_factor_stol > _atom_type_scat_factor_scat_factor_real > _atom_type_scat_factor_scat_factor_imag > _atom_type_scat_factor_rad_type The names are cumbersome. 'factor' strikes me as being redundant and '_scat_factor_scat_factor' overdoes it a bit. How about: _atom_type_scat_type_id _atom_type_scat_type_symbol _atom_type_scat_stol _atom_type_scat_real _atom_type_scat_imag _atom_type_scat_rad_type I'm also very suspicious because only the real and imaginary contributions to the total (both resonant and non-resonant) scattering factors are given names. In practice, for doing electron-density calculations for X-ray diffraction ("Fourier maps") one really needs the resonant (anomalous dispersion) and non-resonant scattering factors to be separated. The non-resonant scattering factor is independent of wavelength. The resonant scattering contributions are dependent on wavelength but, to a very good approximation, independent of sin(theta). > # _atom_type_scat_dispersion_imag > # _atom_type_scat_dispersion_real > # _atom_type_scat_dispersion_source Since you are recategorizing them, and/or perhaps renaming them, the modern tendency is to drop the term 'anomalous dispersion' [what's anomalous about it? Dispersion categorizes a change in property with wavelength but does not identify its physical origin.] and use 'resonant scattering' which clearly identifies its well-understood physical origin. So its worth thinking about: # _atom_type_scat_resonant_imag # _atom_type_scat_resonant_real # _atom_type_scat_resonant_source These names are clear as they identify the resonant contribution to the scattering process in contrast to _atom_type_scat_real and _atom_type_scat_imag which give the total real and imaginary contributions. BMc> loop_ BMc> _atom_type_symbol BMc> _atom_type_oxidation_number BMc> _atom_type_number_in_cell BMc> _atom_type_scat_dispersion_real BMc> _atom_type_scat_dispersion_imag BMc> _atom_type_scat_source BMc> C 0 72 .017 .009 International_Tables_Vol_IV_Table_2.2B BMc> H 0 100 0 0 International_Tables_Vol_IV_Table_2.2B BMc> O 0 12 .047 .032 International_Tables_Vol_IV_Table_2.2B BMc> N 0 4 .029 .018 International_Tables_Vol_IV_Table_2.2B BMc> If the scattering factors are dependent on wavelength and scattering BMc> angle, what exactly do the numbers in the above example refer to anyway? _atom_type_scat_dispersion_real and _atom_type_scat_dispersion_imag refer to resonant scattering contributions only (anomalous dispersion) which are assumed to be independent of sin(theta), and for a fixed, unspecified or default, single wavelength. I understand that does not answer Brian's question. What is saying is that the above loop construction lacks a pointer or link to identify the wavelength for which the resonant contributions are being specified. One of the worst case situations which this construction would hopefully be able to deal with occurs when the absorption edge of the element specified in _atom_type_symbol falls in between the alpha1 and alpha2 wavelengths of the sealed-tube radiation source. eg Re with Mokalpha. > Is it the case that items like the Cromer-Mann coefficicients (which, as I > understand it, are used in an analytic approximation to scattering factors > in a function of lambda and theta) Cromer-Mann coefficients are analytic approximations to the non-resonant contribution to scattering which is independent of wavelength. The C-M coefficients have stol has independent variable. BM> A more distinctive name for such a new category would BM> be ATOM_TYPE_SCATTER; the distinction between ATOM_TYPE_SCAT and BM> ATOM_TYPE_SCATT would be lost to the average user). I agree with you 5000% on that. User understanding is one big problem. Also the typographic difference is tiny and in a rush that final S will be overlooked. Nature has finally resolved the problem of my tearing my hair out over REFLN and REFLNS.

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