[Pw_forum] monoclinic cell definition question

[Jonas Baltrusaitis]

I found discrepancies in how monoclinic cell is defined in the manual and how it's defined in experimental crystalography files:

Namely, 

For P lattices: the special axis (c) is the z-axis, one basal-plane
vector (a) is along x, the other basal-plane vector (b) is at angle
gamma for monoclinic. While it is not clear angle to which axis, later on gamma is defined as an angle between a and b. the problem is that ab is 90 degree angle, not a unique angle, if I visualize the structure with cell axes.

Furthermore, when I open any crystalography file in Mercury, I get for monoclinic cell

alpha 90.00 betta 115.6670(10) gamma 90.00 so gamma is again not a unique angle but rather 90 degree angle. I assume this is not what needs to be entered for monoclinic lattice in celldm(4), but rather a unique angle which in both cases is between a and c, so it has nothing to do with b as defined above

Could anybody help out?

Jonas Baltrusaitis
University of Iowa