Category:Electronic occupancy: Difference between revisions

Within the PAW method there is the occupation [math]\displaystyle{ f_k }[/math] for the plane-wave part and the on-site occupation matrix [math]\displaystyle{ \rho }[/math] that characterize the electronic state. Below we list tags and sections that can be used to influence the occupation, besides the obvious influence of the specific structure and exchange-correlation effects.

Modeling excited states by constrained occupation calculations

The delta self-consistent field (ΔSCF) method provides a practical way to obtain neutral excitation energies within density functional theory (DFT) by explicitly constraining the electronic occupations of selected orbitals. In contrast to linear-response or many-body approaches, ΔSCF directly evaluates the total energy difference between ground and excited electronic configurations from self-consistent calculations. This approach connects closely to experimental observables such as the vertical absorption energy (VAE), vertical emission energy (VEE), and zero-phonon lines (ZPLs), which are key quantities in the optical spectroscopy of point defects in semiconductors and insulators ^[1].

• [math]\displaystyle{ \Delta\mathrm{SCF} }[/math] calculations: example of zero-phonon line calculation of [math]\displaystyle{ \mathrm{NV}^- }[/math] center in diamond

Density-functional theory plus dynamical mean-field theory

Density-functional theory plus dynamical mean-field theory (DFT+DMFT)^[2] is a method that provides a more accurate treatment of strongly correlated materials compared to DFT+U. While DFT+U is computationally much more affordable, it incorporates a static correction for localized electron interactions. DFT+DMFT, on the other hand, goes further by treating these interactions dynamically, capturing frequency-dependent electron correlations. A key feature of DFT+DMFT is that the charge density is updated using the DMFT solution, ensuring a self-consistent feedback between the correlated electronic states and the DFT potential. This not only improves the description of phenomena like metal-insulator transitions and quasiparticle renormalization but also allows for the calculation of spectral properties such as photoemission spectra, transport properties, and total energies relevant to structural distortions. To facilitate DFT+DMFT calculations, VASP provides a general interface to DMFT codes, allowing occupation updates ICHARG=5 via an external file vaspgamma.h5 / GAMMA to update the charge density.

• DFT+DMFT calculations: example of performing DFT+DMFT calculations using the TRIQS software^[3]

References