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Hi dear colleague
I write this thread to ask you advises on what I'm doing now. I'm working on HfO2 monoclinic structure and this is giving me a pain. As you know monoclinic structure has 3 different lattice parameters (a=/=b=/=c) and also Beta angle is not 90.
I referenced those informations from other papers. And with ISIF=6, I relaxed it's structure and got final structure. And here is the problem. To get bulk modulus of this HfO2, I need to change the lattice parameters and see what happen in total energy. But when I change lattice parameters, which one should I change? One at a time? Or all 3 at the same time? What about Beta angle? Plz gives me any clue if you have an experience on things like this. Any comments will be really helpful. Following is my POSCAR. (I have several datas with diffrent lattice parameters and Beta angle)

HfO2
5.1172 5.1722 5.2948
0.987511921226547 0.000000 -0.157544296740485
0.000000 1.000000 0.000000
0.000000 0.000000 1.000000
4 8
selective dynamics
direct
0.275900000 0.041200000 0.2078000000 T T T
0.724100000 0.541200000 0.2922000000 T T T
0.724100000 0.958800000 0.7922000000 T T T
0.275900000 0.458800000 0.7078000000 T T T
0.073000000 0.346000000 0.3320000000 T T T
0.927000000 0.846000000 0.1680000000 T T T
0.927000000 0.654000000 0.6680000000 T T T
0.073000000 0.154000000 0.8320000000 T T T
0.446000000 0.748000000 0.4880000000 T T T
0.554000000 0.248000000 0.0120000000 T T T
0.554000000 0.252000000 0.5120000000 T T T
0.446000000 0.752000000 0.9880000000 T T T

please check if you can find any information of B and its relation to the respective Cijs. If not, I would simply derive B from a series of calculations with different V but preserved ratios of axis lengths and Beta. (this can be done most easily by giving the cell volume as the 2nd line of POSCAR as -V (instead of a,b,c) and fixing the axes ratios and angles via the Bravais matrix. Please choose a small range of V in order to keep the error due to anisotropies of the compressibility as small as possible, I am aware that this prcedure is only correct if the crystal compresses (almost) isotropically upon pressure.
I am not an expert on these things myself, any further suggestion or comment is very welcome !

Hi
I am also confused about the lattice parameters of a noncubic box. I don't know how to convert the lattice parameters (a,b,c and the angles) to the vector lattices. Whould you please me some suggestions? Thanks.