May I know for Castep and Dmol, if i would like to do a cell optimization, can i constraint lattice parameter a and b to be equal (a=b) for materials like ZnO with a P63MC spacegroup, a wurzite structure? I understand in the materials visualizer, i am able to fixed the fractional coordinates and also possible to fixed the lattice parameters, but somehow when I create a supercell which has a P1 symmetry, i can't constraint lattice a=b.

Looking in another way, if I would like to do an a cell optimization on maybe ZnO, should i not constraint a=b and let it optimize freely?