I still have to try that, but it looks exactly like what i was looking for, thanks!

thanks gryphys for all your interesting comments! In the paper for the first structure (MALNAC02, Journal of Chemical Crystallography (1994), 24, p75) it says: "The observed unit cell can be transformed to [the other one/MALNAC]. The present choice interchanges the old a and b axes while the new c axis is chosen to correspond to repeat distance along a shorter diagonal." i am not sure I understand what you mean with "distorted" - the authors do claim that their structure is more accurate, but they do not seem find any qualitative differences...

[-b,-a,b+c] ... sorry i'm not familiar with this type notation, could you explain? also, both structures are xray and according to the CCDC (content of cif files) taken at room temperature, but one (MALNAC) is from 1957 and the other (MALNAC02) from 1994, so a difference in cell volume of about 1 percent should be acceptable/tolerable i believe...

the lattice parameters are indeed different, but the crystal structures are still identical (within exptl error bars) the crystallographers simply made different decisions about how to represent the structure in both cases (think about P21/n vs P21/c) so that is not the issue really - the question is how to rotate one cell so that the content of both cells are aligned/superimposed (again within exptl error bars)...

thanks, QCP looks like a rather useful tool, however, my goal is not to align two structures but to figure out the angles mentioned in the original post, it might be trivial for an expert, but I don;t know how to get these angles from the rotation matrix that a tool like QCP would supply.

thanks ... i've had a look at all three, unfortunately there's no straight forward answer in any of them ... but obviously the basic background is there, so i have to figure it out myself somehow ... this appears to be surprisingly non-trivial ;) even if the two crystal symmetries are fully defined, which they are as i have all lattice vectors for the two cases ... for now i;m doing it by visual inspection, followed by guessing the rotation axis and angles, and then use the matrices from the link i gave in the original post ...

sorry ... i meant P21/n or a .... typo ... and I was talknig about a voigt matrix which is not a tensor at all, but it can still be transformed with a nother matrix to account for a differnt base/reference ... so what i meant can i use the rotation matrix you provided for this purpose?

sorry and one more follow up ... I want to convert C21/a to C21/n (and not the other direction as discussed by you) - would I just use the inverse of the matrix you provided in this case?

thanks for your answer! ... I am not sure though whether it answers the question ... I am looking for a rotation matrix to convert the stiffness tensor (either the 4th rank tensor, or preferably the Voigt matrix), and not just to rotate the structure/coordinates - or are these the same?