ENCUT

ENCUT is a parameter in the Vienna Ab initio Simulation Package (VASP) that controls the plane-wave basis set cutoff energy. It specifies the maximum kinetic energy of plane waves included in the expansion of the electronic wavefunctions. In VASP, the plane-wave basis is built from plane waves with |G|^2 ≤ 2m_e ENCUT / ħ^2, so ENCUT has units of electronvolts. A higher ENCUT yields more plane waves and higher accuracy, at the cost of increased memory and CPU time.

For pseudopotentials and PAW potentials, ENCUT should be set at least equal to the recommended cutoff of the potential file (often expressed as ENMAX). Practically, users perform convergence tests: run calculations at increasing ENCUT values until total energy, forces, and stresses stabilize within a chosen tolerance. Some properties, such as vibrational frequencies or band gaps, may converge more slowly with ENCUT than total energy.

Common practice: for typical elements with standard PAW potentials, ENCUT values in the 400–600 eV range are common; light elements often require 300–450 eV, while heavier elements or hard potentials may need 600–800 eV. Always consult the potential files and the provider's documentation. ENCUT interacts with k-point sampling; a higher ENCUT increases the plane-wave basis size and thus CPU cost, while an appropriate k-point grid ensures convergence of Brillouin-zone sampling.

ENCUT is a central input parameter in VASP calculations, and careful convergence testing is essential to obtain reliable total energies and derived properties.