Modeling charge and energy transfer at defects remains a serious challenge for electronic structure theory. Density functional theory (DFT) reasonable band energies at modest expense, but can be unreliable for strong correlation in localized states. Multireference wavefunction theory can treat strong correlation, but its computational expense makes delocalized states challenging. We have presented a new class of wavefunction-in-DFT methods that project the electron-electron interaction operator onto an active space,  introduce the projected operator into the DFT reference system, then treat the remainder using conventional density functionals. This contribution reports an application to the anionic paramagnetic boron vacancy defect in hexagonal boron nitride. This challenging system combines both delocalized excitations into the conduction band, and a localized and strongly correlated first singlet excitation. Our approach provides reasonable accuracy for both states, by combining a minimal active space with a systematically improvable and double-counting-free DFT correction.

Reimagining density functionals for multireference wavefunction theory: Simulating both strongly correlated and delocalized excited states of a boron nitride defect

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