I'm new to Quantum ESPRESSO and I have read that there are several scenarios when relax calculations are preferred over vc-relax calculations, for example in maintaining vacuum while modelling surface. Is there any other scenario wherein I'd probably use vc-relax over relax. Or can I get away with just a relax calculation? Since vc-relax calculations are computationally more expensive.
'vc-relax' and 'relax' are two quite different functionalities of the PW module. 'vc' stands for 'variable cell', so the cell shape (angles and length) as well as the atomic coordinates are optimized. Both the forces and stress tensor are calculated in 'vc-relax'. 'relax' only does the atomic coordinates optimization, i.e., only the forces are calculated. which tag to choose depends on your system and properties that you are looking for. For example, for phonon calculations, you mush use 'vc-relax' to fully relax the force and stress to get rid of any imaginary frequency issue.