1. First relax ionic position and then do full volume relaxation where you relax" ionic position, cell shape and cell volume".
2. Doing only volume relaxation with increasing plane wave cutoff by 25-30%.
3. Fitting the energy over a certain volume range to an equation of state. First we only volume relaxation (cell and ionic positions are kept constant) and then we sample different volumes around the equilibrium volume. At each fixed volume now you only relax ions and cell shape. Now you plot E-V curve and look at which volume , energy is minimum. ex. for hexagonal system volume scan, a&b lattice parameters are kept fixed while the c axis is scanned and vice versa