To optimize the orthorhombic structure follow the procedure in the following link:
https://wien2k-algerien1970.blogspot.com/2016/09/optimization-of-orthorhombic-structure.html
To study the effect of pressure on lattice parameters (a, b and c ), we proceed as follows:
- we use the equation giving variation of volume as function of pressure
check this link here
V0, a0, b0 and c0 correspond to P= 0 GPa and V1, a1, b1 and c1 correspond to pressure P1.
- We make the value of volume constant and we optimize the c/a and b/a rations according to the previous procedure to get the optimized values of a,b and c .
NB: I couldn't find a similar equation for Birch-Murnagham EOS.
You can check discussion related to the subject in wien mailing list
[Wien] How to get lattice parameter as a function of pressure?
The following articles may be useful
Pressure-induced orthorhombic structure of PbS
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