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 

V(P) = V_0 \left(1+B'_0 \frac{P}{B_0}\right)^{-1/B'_0}. (7)          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

 

Theoretical Predictions of Lattice Parameters and Mechanical Properties of Pentaerythritol Tetranitrate under the Temperature and Pressure by Molecular Dynamics  Simulations