Abstract
The widely used density functional theory (DFT) has a major drawback of underestimating the band gaps of materials. Wannier–Koopmans method (WKM) was recently developed for band gap calculations with accuracy on a par with more complicated methods. WKM has been tested for main group covalent semiconductors, alkali halides, 2D materials, and organic crystals. Here we apply the WKM to another interesting type of material system: the transition metal (TM) oxides. TM oxides can be classified as either with d0 or d10 closed shell occupancy or partially occupied open shell configuration, and the latter is known to be strongly correlated Mott insulators. We found that, while WKM provides adequate band gaps for the d0 and d10 TM oxides, it fails to provide correct band gaps for the group with partially occupied d states. This issue is also found in other mean-field approaches like the GW calculations. We believe that the problem comes from a strong interaction between the occupied and unoccupied d-state Wannier functions in a partially occupied d-state system. We also found that, for pseudopotential calculations including deep core levels, it is necessary to remove the electron densities of these deep core levels in the Hartree and exchange–correlation energy functional when calculating the WKM correction parameters for the d-state Wannier functions.
Highlights
Transition metal (TM) oxide is one of the most important class of materials used in electronics, solar cells, catalysts, batteries, and many other devices
It is well known that the density functional theory (DFT) approximated by local density approximation (LDA)[1] or generalized gradient approximation (GGA)[2] and implemented with Kohn–Sham equations significantly underestimates their band gaps due to the lack of discontinuity in their functional derivatives
Some of the calculations require a large number of conduction bands to yield fully converged results, which can lead to debates about the accuracy of the GW method
Summary
Transition metal (TM) oxide is one of the most important class of materials used in electronics, solar cells, catalysts, batteries, and many other devices. On top of all these, GW calculation can be expensive, especially when a large number of conduction band states are needed or when the system size is large Given this situation, it will be very interesting to find whether there are alternative parameter-free methods to predict the TM oxide band gaps. The integer number of electrons in can yield accurate band gaps on main group covalent semiconductors[21], alkali halides22, 2D materials[23], and organic crystals[24] For these tested materials, the accuracy of WKM is similar to that of GW method. As a result, according to Janak’s theory[28], the total energy difference is the same as Kohn–Sham orbital eigenenergy To overcome this problem, we added an electron into a localized Wannier function instead of the extended Kohn–Sham orbitals.
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