The partner of the Hubbard U: interpretation of Hund's J in DFT+U methods, linear-response calculations for J, and the results to expect
School of Physics, SFI AMBER Centre, and CRANN InstituteTrinity College Dublin, The University of Dublin, Ireland
Hubbard U parameters are now understood as quantifiers for the self-interaction error associated with selected subspaces in approximate DFT, within the approximate framework of DFT+U. Here I will review the lesser-known counterpart of the Hubbard U parameter, the so-called Hund's J, the calculation and use of which has to date been rather more sparse. I will describe how Hund's J can be similarly interpreted and calculated, but as a measure instead of subspace static correlation error (poor description of specific multi-reference effects) in DFT. Practically, I will show how Hund's J can be readily calculated like the Hubbard U using finite-differences linear-response [1,2], in certain cases as a cost-free by-product of U . This will highlight how J is not necessarily insignificant in closed-shell systems, as it might be assumed. I will highlight that the DFT+U+J functional introduced in Ref.  has recently yielded promising results for several oxides including MnO , TiO2 (rutile and anatase) , Cr2O3, and NiO . I will explain how parameter self-consistency conditions can be modified to incorporate Hund's J .
 G. Moynihan, Ph.D. thesis, Trinity College Dublin (2018) http://hdl.handle.net/2262/82220
 E. B. Linscott, D. J. Cole, M. C. Payne and D. D. O’Regan, Phys. Rev. B 98, 235157 (2018).
 O. K. Orhan and D. D. O'Regan, Phys. Rev. B 101, 245137 (2020).
 B. Himmetoglu, R. M. Wentzcovitch, and M. Cococcioni, Phys. Rev. B84, 115108 (2011).
About the speaker
Prof. David O’Regan completed his PhD at the Cavendish Laboratory, University of Cambridge in 2011, after which he carried out post-doctoral research first there, and then at the École Polytechnique Fédérale de Lausanne (EPFL). He is currently an Assistant Professor in the School of Physics, where he serves as the Director of Teaching and Learning (Undergraduate), with an affiliation to CRANN.
His research concerns development and innovation in the accurate and computationally increasingly routine quantum-mechanical description of the material world, specifically practical electronic structure simulation for solid state and molecular systems, together with theory and technique development in condensed matter physics and quantum materials discovery. He places an emphasis on materials for ICT and renewable energy. Additionally, he has a complementary long-term research interest in more abstract corrective functional development in density functional theory and in many-body theoretical spectroscopy methods.
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