Hi Anatoly, I agree that a) it's unfortunate that Artemis is too clever in noticing that a P1 space group has some symmetry and prevents the Feff calculation when this happens. I'd suggest a space group called 'Cluster' or 'Molecule' or something that would indicate that there should be no space filling procedures done, and that all atoms in the molecule/cluster are specified, but still let you enter atomic coordinates as relative to a 'lattice constant'. It is possible to just enter the xyz coords in the feff.inp, but Atoms/Artemis does such a nice job at filling out the rest of feff.inp that using a P1-like space group is a very convenient choice. b) using a = 99.9, b=100.0, c=100.1 is a good way to fool Artemis/Atoms.
For molecular substances, the accuracy in atomic coordinates that are obtained by one of the three most popular methods (e.g., in Protein Data Bank, the sources are X-ray crystallography, solution NMR, or theoretical modeling, here is a good review: http://www.rcsb.org/pdb/experimental_methods.html) is much worse than 0.1 A.
That seems a little unfair. Certainly most macromolecular (>5kD) diffraction data as is typically found in the PDB often has a resolution worse than 1A. But powder diffraction measurements and theoretical modeling of crystallized small molecules (and the definition of 'small' seems to be growing) and clusters often have precisions better than 0.01A. If 0.1A really were the accuracy needed, then you could use a = 90, b=100, c=110 in your example. That would give a very noticeable difference in any decent powder diffraction data and in the EXAFS. OTOH, using '99.9, 100.0, 100.1' would be more challenging to see with powder diffraction and very difficult to see with EXAFS. In both cases, the distances could easily be refined from that "off by 0.1%" starting point. For EXAFS at least, the same would not be true if the starting distances were off by 10%. Anyway, paraphrasing Juraj's suggestion, using '99.99, 100.00, 100.01' will certainly be OK and is not any harder to type than '90, 100, 110'. --Matt