Written by Makena Kaiman ([email protected])

VASP (Vienna Ab initio Simulation Package) is a commercial software created to study materials at the atomic level. It helps us predict how atoms and electrons behave in different systems, simulate physical properties (energy, structures, or movement), and virtually model materials.

Using VASP gives us access to a lot of different data points that become really useful for first-principles calculations. For example, we can obtain accurate data about the electronic structure, charge density, and spin states of the Nitrogen-vacancy center defect. This information can help us predict how the NV center behaves under various conditions and in great detail.

The 4 essential VASP input files

To run a simulation with VASP, you will always need to have these four input files. These files define the system you are going to simulate and run calculations with. It’s important to try and familiarize yourself with what these four files contain and why they’re critical for running a VASP job.

POSCAR - This file describes the atomic structure of your system. It contains the lattice vectors, the types of atoms, and their positions. It’s like a “blueprint” of the material for your simulation. See the VASP Wiki POSCAR page for more information.

INCAR - This is the main control file that sets all of the calculation parameters, such as the type of calculation (energy, relaxation, etc), convergence criteria, and electronic settings. It’s like an “instruction manual” for your simulation. See the VASP Wiki INCAR page for more information.

KPOINTS - This file tells VASP how to sample the Brillouin zone. It essentially controls how detailed your simulation is when it comes to the electronic structure of the material. More k-points means more detail, but it also means more computational time. See the VASP Wiki KPOINTS page for more information.

POTCAR - This file holds the pseudopotentials for each element in your system. It essentially tells VASP how to treat each element’s atom in your system, and more specifically, how the electrons interact with the nuclei. See the VASP Wiki POTCAR page for more information.

3 important VASP output files

Once you submit the four essential input files and run them in your job, VASP performs the calculations and generates a variety of output files that contain details about your system’s properties and results. Although it produces a lot of output files, we’re just going to focus on these three important ones for now.

OUTCAR - This is the most detailed output file. It gives us a lot of information, like the initial calculation settings, forces and energies, electronic structure information, magnetic moments, stress tensor and pressure, CPU time & resources, and convergence info (which is super important). When you run a job, a lot of times the first thing you’ll check is whether or not your calculations converged on OUTCAR, so it’ll be good to familiarize yourself with checking for convergence at the bottom of the OUTCAR file. See the VASP Wiki OUTCAR page for more information.

CONTCAR - This file stores the final atomic positions after the relaxation of your system. It’s important to note that it can be used as a new POSCAR file to continue simulations, especially if the system hasn’t converged yet. See the VASP Wiki CONTCAR page for more information.

CHGCAR - This file stores the detailed information about the charge density of your system after the simulation has run. See the VASP Wiki CHGCAR page for more information.