# Real space projection vs reciprocal space projection in DFT calculations

When doing VASP calculations for large cells, we get a warning:

"You have a (more or less) 'large supercell' and for larger cells it might be more efficient to use real space projection operators. So try LREAL= Auto in the INCAR file. Mind: For very accurate calculation you might also keep the reciprocal projection scheme. (i.e. LREAL=.FALSE.)"

Why is using real space projection operators efficient for large supercells, and why are they less accurate?

This aliasing error is different than the eggbox / wrap around errors that occur with lower PREC values and can be compounded on top of it when considering very small energy differences. For large cells that need to be evaluated within meV, you should relax with LREAL=Auto, then LREAL=False if you expect this error will make a difference.