Research

 

We are a theory group bridging three fields, chemistry, physics and material science. Our motivation for research steams from the recent progress in experimental chemistry that enabled manufacturing new materials that can be used in a variety of industrial applications.


Semiconductors, heterostructures and thin layered films are used for energy harvesting and solar sells. Newly emerging “electron correlation” devices made out of transition metal oxide heterostructures and metallic surfaces with deposited magnetic molecules can be used for  signal conversion, nonvolatile memory and spintronic devices.


Currently, this experimental progress poses many questions to our theoretical understanding.  These questions should be answered in materials or molecules by design solutions using a combination of modeling and theory to support experiment.

In our group, to tackle these important questions we are developing controlled, reliable, and systematically improvable methods that describe correlation effects and are able to treat solid and large molecules realistically.


Our research interests can be summarized by the following scheme

We aim to develop new ab-initio methods that can provide insight into basic understanding of chemistry and physics of solids and molecules as well as be used in many important applications for the energy research.



Here is the the list of topics we find currently interesting.

Weakly correlated molecules and solids

Our current interest is in developing novel systematically improvable methods applicable to weakly correlated solids that will allow to predict many experimentally relevant quantities such as atomization energies, heats of formation and band gaps. The band gap is a defining feature of Si, GaAs, GaSe and Ge - all semiconductors used to make  heterostructures and solar cells. Such new methods could be also used in molecular systems to predict energies of spin states that are relevant for our understanding of the functioning of the active centers of biological systems.


Many methods designed for calculating weakly correlated systems exist, most notably the Density Functional Theory (DFT) method. However, despite the great success of DFT in predicting lattice constants and the geometrical structure of solids, DFT with local and semi-local functionals does not give accurate predictions of atomization energies, heats of formation or band gaps. Additionally, DFT does not offer a systematic way of improving its results.


Error of LDA (squares)  and GLDA WLDA (circles) in calculating fundamental gaps for different sp compounds.

Strongly correlated molecules

Strongly correlated molecules containing d- and f-electrons that occupy localized orbitals have many biological and industrial applications. Molecules containing transition metal - oxygen or - sulphur complexes are in active centers of many metalloproteins such as ferredoxine, Coenzime Q or nitrogenase. Transition metals nano-particles are novel catalysts that may become more efficient than traditional catalysts in car industry. Molecular magnets containing many transition metal and oxygen bonds have potential applications to quantum computing and high-density information storage.

Unfortunately, strongly correlated molecules are not described well by one-electron approximations such as DFT or by low orders of perturbation theory that are used for treatment of weakly correlated molecules. Consequently, they pose a new challenge for theoretical approaches.

We are interested in developing theoretical approaches that combine Dynamical Mean Field Theory (DMFT) and weakly correlated approaches based on Perturbation Theory (PT) or DFT to obtain an accurate energetics for these strongly correlated molecules.

Molecular Magnet

Mn12-acetate

Ferredoxine

Fe4S4 cluster

Surface chemistry and catalysis

Surface chemistry is computationally demanding because it needs to interface solid state and molecular chemistry. Theoretical insight into surface reactions is necessary to describe a large number of surface phenomena including corrosion, electrochemistry, and heterogeneous catalysis.

At present, the adsorption processes on metallic surface can be modeled by DFT or Random Phase Approximation (RPA). The DFT results are typically functional dependent in prediction of the surface bonding site. RPA is a more suitable method than DFT, however, due to its perturbative nature, it cannot describe bond breaking between the molecule and the surface. Transition metal oxide surfaces (such as NiO) form another common group of catalysts. These cannot be properly described by any of the current methods.

We are interested in using the QM/QM embedding approach to calculate the differences in adsorption energy between different sites on the surface. We want to treat the orbitals of the absorbed molecule with the DMFT approach while the rest of the system is treated with some less expensive computational method.



“CO puzzle” almost all methods predict incorrect adsorption site of CO molecule on Pt, Au, Cu, Rh surface.

Modeling of Kondo resonances and molecular conductance

Since the first proposal of a molecular device in 1974, the field of molecular electronics has been an area of a vigorous research. The underlying motivation for further theoretical progress is the understanding of electron transport, which in turn can provide further insight into molecular device design. Currently, the majority of conductance calculations are using DFT which is effective one-electron approach incapable of properly describing electron correlation. 

We are interested in including the description of electron correlation beyond DFT in the common conductance formalism. This will allow us to study the temperature at which the Kondo resonances (crucial for functioning many of the devices) are appearing.

We plan to base this molecular conductance scheme on the QM/QM embedding approach were the device region is described at a higher level of theory than DFT.

“Manipulating Kondo Temperature via Single Molecule Switching”

Iancu et al., Nano Lett., 2006,6, 820-823