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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.

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  • $\begingroup$ If you want to obtain strain-free lattice constants, you will have to choose vc-relax. This way the lattice parameters and cell shape become degrees of freedom in the relaxation procedure. The atoms inside the cell are relaxed as well. On the other hand, if you want to investigate behaviour of say a molecule being adsorbed on a surface, you will have to choose relax. 'Relax' basically keeps the initial lattice inputs constant and therefore simulates the forces between the surface and the molecule. $\endgroup$ – Xivi76 Sep 26 at 1:10
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'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.

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  • $\begingroup$ Phonons are well-defined even under a finite pressure or stress, so not convinced that phonons are the best example of the need to use 'vc-relax'. What you definitely need for phonons are zero forces on the atoms in your reference structure, which can be obtained with 'relax'. $\endgroup$ – ProfM Sep 24 at 6:24

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