Jon wrote, >On a related question - is there a really good reason for putting >quadratic products into the least squares matrix rather than the real >space unit cell parameters? I haven't picked up yet on why the >derivatives >cannot be converted from w.r.t A* etc to w.r.t a etc. Is it just a time >saving? With regards to magnetic structures there is another 'piece of time saving', or rather an unnecessary complication, to clear up, and that is the requirement in most refinement packages to enter the entire magnetic unit cell. The natural language of magnetic structures is written in terms of the magnetic propogation vector, k. This simple concept expresses all the translation symmetry of a magnetic structure. If a magnetic cell is described as having a propagation vector k = (0 1/2 1/2) rather than cell parameters a, 2*b, 2*c the intensity of the magnetic reflection can be calculated only using the atoms in the crystallographic (zeroth) cell. This rather painless change would mean that programs such as GSAS could refine commensurate and incommensurate structures alike (***finally***). Description in terms of cell parameter can also lead to confusion because occasionally people try to use 'cell transformation' relations, I recently saw a magnetic structure that was 'described' using the lattice vectors: a_mag=1/2(a-c) b_mag=b c_mag=a+c Which, quite frankly, is a bit of a mess... Also, as the zeroth magnetic cell is then the same as the crystallographic, the lattice parameters are identical and there is no need for the problem of quadratic constraints. :-) -Andrew ____________________________________________________________________ Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
