Category:Electronic ground-state properties: Difference between revisions

At the end of an electronic minimization, VASP has obtained a converged set of orbitals. From the orbitals, we compute the corresponding density via k-point integration. The orbitals and the density reveal important insights into material properties and are often the first step towards analyzing and understanding a material.

Properties of orbitals

For practical purposes, one is most interested in the energy eigenvalue of the orbitals and their local projections. Consider first the eigenvalues: Counting all orbitals with eigenvalues in a certain energy interval yields the density of states (DOS). Often looking at the DOS can provide valuable insight into the electronic properties of a material and is therefore often the first step to start towards understanding a novel material. The band structure contains even more details about the electronic eigenvalues by resolving them with respect to the Block vector k. Usually, one uses high-symmetry paths in the Brillouin zone for band structures. They make it easy to recognize fundamental and direct bandgaps of the material. Alternatively, VASP reports these to the OUTCAR file with the verbosity controlled by the BANDGAP tag. The electronic eigenvalues determine the occupations of each orbital in conjunction with the settings for ISMEAR, SIGMA, and EFERMI.

Next, we consider the projections of the orbitals which you activate with the LORBIT tag. VASP projects each orbital onto functions with defined angular momentum near each ion. This site projection augments the data produced by DOS and band structure calculations. An easy way to visualize these projections is with py4vasp. In addition, the projections describe how much charge has a particular angular momentum and spin near an ion. This serves as a good approximation for the magnetic structure of the system.

Properties on the grid

The charge density is defined on the FFT grid and results from a sum over bands and k points of all occupied orbitals. For collinear (ISPIN=2) and noncollinear (LNONCOLLINEAR=T) calculations, it contains additionally the magnetization density. VASP stores the charge density in the CHGCAR file and can use it to restart a calculation (ICHARG≥10) which is particular relevant for non-self-consistent calculations like band structures.

Most of the time, the total charge density is not specific enough to get insight into material properties. For this reason, VASP offers the possibility to create band-decomposed charge densities. Selecting a specific band index or k point can shed light e.g. on the localization of defects. Use this feature also to compare to experimental scanning-tunneling-microscopy (STM) images. The charge density near the surface in the vicinity of the Fermi energy is a good first approximation.

Another quantity computed during the optimization is the total potential. If you want to analyze the potential, use the WRT_POTENTIAL tag to select which potential is written to file. There is also the legacy flag LVTOT when you only need the total potential. A common use case is to inspect planar averages parallel to surface or interfaces. VASP will automatically compute this average depending on the IDIPOL setting. Compute the difference of the average potential and the Fermi energy to get the work function.