 Another way to mitigate the effect of Pulay stress on the equilibrium cell shape is to calculate the energy at different lattice vectors with a fixed energy cutoff.Similarly, the error occurs in any calculation where the basis set explicitly depends on the position of atomic nuclei (which are to change during the geometry optimization).In this case, the Hellmann–Feynman theorem – which is used to avoid derivation of many-parameter wave function (expanded in a basis set) – is only valid for the complete basis set.