Dr Aron Cohen

Our research focuses on the understanding and development of electronic structure methods, particularly that of density functional theory (DFT). DFT is an important computational tool with widespread use throughout many areas of chemistry and physics. The beauty of DFT is that it is an exact theory, yet conceptually and computationally simple. However there is one term that has to be approximated, the exchange-correlation functional, and the validity of any DFT calculation relies completely upon it.

Our work focuses on understanding and improving currently used approximations (such as LDA or B3LYP). It has been understood in the literature that these functionals can suffer from a delocalisation error, as exemplified by the incorrect dissociation of H₂⁺, and a static correlation error, as shown by the errors in stretching of closed-shell H₂. We now understand that these two errors should not be considered separately but rather as just two symptoms that come from the violation of a unified exact condition. The understanding of this exact condition of the exchange-correlation functional, even for a system as simple as the hydrogen atom, has lead to a much deeper understanding of the derivative discontinuity, strongly correlated systems and Mott insulators.

We are beginning to explore the consequences of this and other rationales to address many of the outstanding problems of DFT. We are currently interested in investigating and understanding the effects of the exchange-correlation functional for many systems including organic reactions in chemistry, excitation energies of molecules, polarisabilities of polymers and band-gaps and magnetic properties of solids.

Selected Publications

Landscape of an exact energy functional. Aron J. Cohen and Paula Mori-Sánchez Phys. Rev A 93, 042511 (2016)

The derivative discontinuity of the exchange-correlation functional. Paula Mori-Sánchez and Aron J. Cohen  Phys. Chem. Chem. Phys. 16, 14378 (2014)

Challenges for Density Functional Theory. Aron J. Cohen, Paula Mori-Sánchez and Weitao Yang Chem. Rev. 112, 289 (2012)

Revealing Noncovalent Interactions. Erin R. Johnson, Shahar Keinan, Paula Mori-Sánchez, Julia Contreras-Garcia, Aron J. Cohen and Weitao Yang J. Am. Chem. Soc. 132, 6498 (2010)

Discontinuous Nature of the Exchange-Correlation Functional in Strongly Correlated Systems. Paula Mori-Sánchez, Aron J. Cohen and Weitao Yang Phys. Rev. Lett. 102, 066403, (2009)

Insights into current limitations of density functional theory. Aron J. Cohen, Paula Mori-Sánchez and Weitao Yang Science 321, 792, (2008)