# Real space projection vs reciprocal space projection in DFT calculations

Matter Modeling Asked on August 19, 2021

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?

When doing calculations in real space, there are aliasing errors that can occur. This can result in slightly incorrect energies with respect to small shifts. By working in real space, the calculation of projector operators will scale closer to the number of atoms rather than the number of plane waves since they can be evaluated within a sphere around the atom. The real space method becomes more favorable as the number of atoms increases, which is why you get this warning for large cells.

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.

Correct answer by Tristan Maxson on August 19, 2021

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