Overview

The INCAR: this contains most of the input flags.
The POSCAR: this contains the input structure.
The KPOINTS: this contains details about the k-point grid.
The POTCAR: this contains the pseudopotential details.

INCAR

Please do not try to memorize all the INCAR flags. This is merely for ease of reference. That said, after some time using VASP, you will end up knowing more about these flags than you would ever wish.

 = "Very important; must learn for running your first calculations."

 = "Reasonably important but not needed right way; learn after you've got the basics down."

 = "Fairly specialized. Come back to this later."

Level of Theory

•  ENCUT: this sets the plane-wave kinetic energy cutoff. The higher the value, the more precise your calculations will be at the expense of higher computational cost. At the top of each element's POTCAR file is a parameter named ENMAX. The value of ENCUT , if not explicitly set, is the largest ENMAX value across the elements in your system. In practice, you should set ENCUT to at least 1.3 times this maximum ENMAX value for any structure relaxation involving changes to the cell volume in order to minimize artificial stresses.
• Rule of thumb: Try at least 520 eV to start. In practice, you should ensure this is converged.
•  GGA: this sets the exchange-correlation functional (i.e. the "level of theory"). This has the largest impact on the results of the calculation.
• Rule of thumb: Try GGA = PE first (i.e. the PBE functional), particularly if you are just learning how to use VASP.
• If using ASE, the xc keyword can be used to set the functional as described in the  documentation . It is often more convenient since it will auto-set all relevant flags for that functional (e.g. xc = "pbe" will auto-set GGA = PE).
•  IVDW: this sets any van der Waals correction scheme. Grimme's DFT-D methods are common in molecular systems where vdW corrections are particularly important.
• Rule of thumb: For typical bulk solids, don't need to set it unless you have reason to. For systems with notable vdW interactions, try IVDW = 12 first.
•  LDAU/ LDAUJ / LDAUL / LDAUPRINT / LDAUTYPE / LDAUU: these arguments specify details related to the DFT+U method, which corrects so-called "self-interaction errors" of GGA functionals. If you are new to DFT, do not worry about these parameters yet.
•  LHFCALC / HFSCREEN: these methods specify details related to the use of hybrid functionals. Like DFT+U, this can also correct common "self-interaction errors" of GGA functionals.
•  METAGGA: this parameter defines a particular meta-GGA functional, which is generally more accurate than a GGA functional but less accurate than a hybrid functional.
• If using ASE, the xc keyword will take care of this flag.

SCF

•  PREC: specifies the precision used.
• Rule of thumb: Generally set this to Accurate.
•  EDIFF: this sets the SCF convergence tolerance (in eV). The lower the value, the more precise your energies and forces will be.
• Rule of thumb: Try 1e-5 to start.
•  ALGO: this sets the SCF convergence algorithm.
• Rule of thumb: Try Fast to start. If you have problems, switch to All .
•  ISMEAR: this sets how the partial occupancies are treated (i.e. the smearing method) to smooth out discontinuities near the Fermi level. There are a lot of subtle rules about what value to set for what material as laid out in the VASP manual, which our software stack will try and set for you where appropriate.
• Rule of thumb: When in doubt, ISMEAR = 0 is a good bet with a small value of SIGMA (0.01 - 0.05).
•  LASPH: this defines whether non-spherical contributions are included in the energy and forces. Do not mix calculations with True and False for LASPH.
• Rule of thumb: Always set this to True to make your life easier.
•  LREAL: setting this value to Auto can speed up calculations for large systems but can reduce the quality of the energy and should always be set back to False for computed properties of interest.
• Rule of thumb: Always set this to False (i.e. the default) to make your life easier, even when VASP complains.
•  SIGMA: specifies the smearing parameter (e.g. width). Like ISMEAR, this is very material-dependent.
• Rule of thumb: When in doubt, set this to a small value (0.01 - 0.05) along with ISMEAR = 0.

Geometry Optimization

•  EDIFFG: this sets the maximum net force (in eV/A) for a structure to be considered converged during a geometry optimization. The lower the value, the closer to the true local minimum your structure will be. Note that a negative sign should be used by convention (e.g. EDIFFG = -0.03).
• Rule of thumb: Use a value of -0.03 or tighter (e.g. -0.01).
•  ISIF: this defines which degrees of freedom should be modified during the structure relaxation. ISIF = 2 indicates that the positions are relaxed at fixed cell volume. ISIF = 3 indicates that the positions and cell shape/volume are simultaneously relaxed.
• Rule of thumb: Use ISIF = 3 for cell relaxations and ISIF = 2 otherwise.
•  NSW: specifies the maximum number of geometry optimization steps to consider.
• Rule of thumb: Set to an arbitrarily large value, e.g. 200.
•  IBRION: this is the geometry optimization algorithm.
• Rule of thumb: Try IBRION = 2 (conjugate gradient) to start.
•  ISYM: this defines whether symmetry constraints should be accounted for.
• Rule of thumb: Set to 0 for geometry optimizations to ensure the symmetry is allowed to change, unless you are intentionally looking to model a given symmetry.

Magnetization

•  ISPIN: this sets whether the calculation should be run without or with spin-polarization. If the system has unpaired electrons, it should be run with spin-polarization.
• Rule of thumb: When in doubt, start with ISPIN = 2 to see if there are favorable magnetic states. If there aren't, the default value of ISPIN = 1 is fine.
• If using ASE, setting atoms.set_initial_magnetic_moments() will automatically toggle ISPIN accordingly. Don't directly specify ISPIN in this case.
•  LORBIT: this parameter specifies how much information related to magnetization is written out to the OUTCAR file.
• Rule of thumb: Always set this to LORBIT = 11 to make your life easier.
•  MAGMOM: this parameter specifies details related to the initial guess for the magnetic moments.
• Rule of thumb: Initialize each transition metal with a high-spin magnetic configuration as a first attempt. Then go back and try other magnetic configurations, if applicable.
• If using ASE, setting atoms.set_initial_magnetic_moments() will automatically toggle MAGMOM accordingly. Don't directly specify MAGMOM in this case.
•  NUPDOWN: forces a particular net number of unpaired electrons.

File I/O

•  ISTART: this defines if the VASP calculation should restart from a pre-existing wavefunction. If a WAVECAR is present in the same directory, VASP will automatically try to restart from it.
• Rule of thumb: No need to change the default value in most cases.
•  LWAVE: this parameter specifies whether the wavefunction is written out to the filesystem. This is especially useful for restarting a calculation but is quite large.
•  LCHARG: whether to write out the charge density to the filesystem. This is useful for post-processing but is quite large.
•  LAECHG: this specifies whether AECCAR files are written out, which are used by certain post-processing codes. Never use LAECHG if NSW is not set to 0.
•  NEDOS: describes how many points are written out in the density of states output file (DOSCAR).

Miscellaneous

•  EFERMI: VASP recommends generally just setting this to MIDGAP. That will yield slightly more consistent Fermi energies (and, as a result, band gaps) but otherwise changes nothing.
• Rule of thumb: Always set this to MIDGAP to make your life easier if you are using VASP 6.4+.
•  NCORE: this parameter can be tuned for a specific machine and system to speed up calculations by parallelizing over orbitals. The only way to know which value is best is to try them out! It should generally be used in place of NPAR.
•  KPAR: similarly, this parameter tells VASP how to parallelize the k-points. It only makes sense to use if there are many k-points, as a result.

POSCAR

from ase.io import read, write

atoms = read("/path/to/my.cif")
write("POSCAR", atoms, sort=True)

from pymatgen.core import Structure

structure = Structure.from_file("/path/to/my.cif")
structure.to(filename="POSCAR")

KPOINTS

from pymatgen.io.vasp.inputs import Kpoints
from pymatgen.core import Structure

structure = Structure.from_file("POSCAR")
kpts_obj = Kpoints.automatic_density_by_vol(structure, 100)
print(kpts_obj.kpts)

POTCAR