Description of Input Files

Here, a short introduction is given to the general contents and layouts of the four main input files that are needed for every VASP calculation (depending on the type of the calculation, other files might be needed as well, see below). These are INCAR (containing keywords), POSCAR (containing the atomic coordinates and unit cell geometry of the system), POTCAR´ (containing the PAW potentials for the description of atomic cores) and KPOINTS` (containing the setup for the integration of recriprocal space). The given files are for an example single point of calculation of a liquid metal surface system with an ethylene molecule adsorbed at the surface, their general structure can however be translated to most other systems.

1. INCAR

The keywords for the calculation must be given as upper case identifiers, followed by a space, an equal sign, a space and the value of the variable: WORD = VALUE. It is advisable to add most of the following keywords to each calculation (although there exist default values for most of them, if they are not listed in the INCAR file) and think about their values for the current system.

• SYSTEM = [name] Identifying name for the calculation
• PREC = Normal Sufficient for molecular dynamics trajectories and most geometry optimizations. For energy or property calculations (DOS, band structure, STM, ...), PREC = Accurate should be used.
• GGA = PE The usual PBE GGA DFT functional (used for most of our applications)
• NCORE = [number] Number of CPUs per orbital, should always be a divider of the available number of CPUs per node on which you do the calculation (e.g., the node has 72 cores, NCORE = 18 could be a good choice). For very small systems, strange convergence issues (such as subspace is not hermitian! or no DFT gridpoints available!. This is mostly due to a too large number of CPUs. Decrease the total CPU number (and NCORE) in such a case!
• ISPIN = 1 or 2 If the calculation shall be done with spin polarization (ISPIN = 2) or without (ISPIN = 1). For systems with magnetic elements (e.g., metal surfaces) or open shell structures (during reaction paths or radicals), spin polarization must be used. If the electronic properties of the current system are now known well, spin polarization should be applied first as a test. If in the standard output mag= 0.0000 appears, the spin polarization is zero and the cheaper ISPIN = 1 mode should be used.
• MAGMOM = [natoms]*[value] If ISPIN = 2 is set, for each atom, an initial magnetic moment should be given. The format must be written with explicit * sign, where the atoms in the POSCAR must be covered all (e.g.,40*2.0 for 40 atoms). The default value, is the keyword is not given, is always 1.0 for each atom. For more complicated setups, the magnetic moments might be either deduced from chemical knowledge or from literature references.
• NELM = 300 Maximum number of electronic SCF steps, 300 should be sufficient for most cases (the default value of 60 is sometimes not sufficient for larger or electronically challenging systems).
• NELMIN = 4 The minimum number of electronic SCF steps; might be raised if geometry optimizations cause problems (which can sometimes be mitigated by this increase), e.g., to 8.
• ENCUT = [value] The plane wave cutoff in eV. For static calculations, it is sufficient to choose the largest ENMAX value of all elements in the POTCAR file. If cell parameters are relaxed, the ENCUT value should be 25-30% higher. For geometry optimizations of complicated surface adsorbates, it proved be useful as well to raise the ENCUT value, e.g., to 600 eV. For calculation of adsorption energies, all three systems (substrate, adsorbate, adsorbate+substrate) need to have the same ENCUT value!
• EDIFF = 1E-07 The electronic convergence criterion, energy difference in eV between two cycles. (1E-05 is VASP default, but we are mostly using stricter values)
• LREAL = Auto Evaluation of projection operators in real or reciprocal space. The given option should be the best for systems with more than approx. 30 atoms. For small systems, LREAL = .FALSE. can be used. VASP will give a warning, if the used scheme is not optimal for the chosen system.
• ALGO = Fast Which kind of SCF algorithm is used (Davidson or DIIS). For molecular dynamics of many systems, Very Fast (only DIIS) will be enough, in most other cases, Fast (Davidson for the first few SCF cycles, then switching to DIIS) is well suited. If convergence issues Normal (only Davison) often helps, although being slower.
• ISMEAR The choosen smearing method to enable the integration of electronic bands. The choice of the currect smearing scheme is not trivial, details can be found in the VASP Wiki. Methfessel-Paxton smearing (first order): (ISMEAR = 1) should be used for metallic systems (with or without nonmetallic compounds such as adsorbates into them). Gaussian smearing (ISMEAR = 0) should be used for insulators or molecules or systems with unclear electronic properties. Tetrahedron smearing with Blöchl corrections (ISMEAR = -5) should be used for single point calculations with high accuracy (e.g., for DOS, but not for band structures!). Fermi smearing (ISMEAR = -1) can be used for molecular dynamics trajectories at a certain temperature.
• SIGMA = [value] Width of the electronic smearing in eV, depends on the smearing method. Methfessel-Paxton: 0.1-0.2; Gaussian: 0.03-0.05; Tetrahedron: no SIGMA needed; Fermi: 0.05-0.7 (depends on the temperature during the MD).
• IVDW = 12 Grimme D3 dispersion correction, should be turned on for surface-adsorbate interactions etc., essentially no overhead. Note: For adsorption energy calculations, the dispersion correction need to be turned on also for the clean metal surface, even if there is chemically no dispersion relevant! Else, the energy scale is not the same!
• LWAVE = .FALSE. No wavefunctions written to WAVECAR. Should be the default in order to limit memory pollution. WAVECAR files are needed, e.g., for STM simulations and implicit solvent calculations.
• LCHARG = .FALSE. No charge densities are written to CHGCAR or CHG. Also default for memory saving, only needed for band structures, Bader charges, STM pictures, charge densities, etc.
• ISYM = [number] Which kind of symmetry will be used in the calculation. For molecular dynamics simulations, ISYM = -1 should be set in order to explore the full possible configuration space and for frequency calculations to avoid strange errors.