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)
• 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]