[QE-users] vc-relax for orthorhombic systems

[Lorenzo Paulatto]

> Could you please rewrite the last sentence?

During the vc-relax procedure, the Fourier transform grid is kept fixed 
to avoid Pulay stress. The condition to incldue a certain plane wave in 
the basis set is hbar |G|^2 < Ecutwfc, and G is an integer linear 
combination of the reciprocal basis vectors b1, b2, b3. These b_i 
vectors change when the volume changes, which means that at the end of 
the relax, the basis set is not consistent with the cutoff and may be 
anisotropic. If you repeat the relax from scratch using the previous 
final configuration as the initial one, you may get different results. 
Rinse and repeat until you don't.

cheers



-- 
Lorenzo Paulatto - Paris