Anshuman Kumar (then an EP alumnus, PhD student, now Professor at IITB) coming to the rescue for me again: “I am providing two approaches, which have generally been mentioned in the other answers. The only place where my answer differs is that it is specific to PWscf. Two methods:
1) Brute force: Run scf calculation for different lattice parameters. Plot total lattice energy as a function of the lattice parameter Look for minimum energy point, whose abscissa will give the optimized lattice constant
2) Smart way a) In the &control block in you input file, set: calculation=’vc-relax’, b) In the &ions block of the input file, set ion_dynamics = ‘bfgs’ c) In the &cell block, set cell_dynamics = ‘bfgs’ cell_dofree = ‘xyz’
Thats all.”