Overview
We are engaged in implementing and developing electronic structure methodologies to study several types of systems:
1. Molecular Conductance
2. Hydrogen fuel economy
3. Inorganic reaction mechanisms
4. Excited states studies of extended systems and biological systems.
1. Molecular conductance
Our major research effort is on furthering the state of the art quantum mechanical description of molecular conductivity. In one aspect of the research we are engaged in implementing novel schemes for modeling molecular transport. Most high level computational models of molecular electronic current are based on the self consistent solution of the non-equilibrium Green's function (NEGF). Our efforts are based on deriving schemes based on time-dependent (TD) DFT as an alternative to the NEGF approach. In our approach, we pursue 2-variable TD expansions to gain a high quality description in transport studies. This involves efficient incorporation of self-energy (SE) terms in TD expansions. Tractable and high quality SE expressions are crucial for obtaining reliable description of the molecule coupling to the bulk under the effect of potential bias. These schemes utilize the TDDFT expansions for studying transient effects in molecular devices as the system reaches steady state upon application of a potential bias or even under the influence of a laser field. In addition, several important applications of molecular devices are pursued. Some examples include the spin-dependent transmission in ligated porphyrin molecules, molecular field-effect transistors, transport in chemical sensors and studies of fundamental modeling issues. More recent studies include elaborating experimental successes in metal recognitions functionalities as recently reported by Tao and coworkers and schemes for developing molecular rectifiers.
Sample: Research nugget on modeling structure-function relation of a molecular based device.
Presentation Poster 1
Presentation Poster 2
Porphyrin Study: Spin coupling dependent transport
We consider, for illustration, the spin dependent transmission of an iron ligated porphyrin system. In this system strong dependence of the electronic transmission on the Spin coupling is demonstrated. The spin coupling of a ligated porphyrin depends on its axial ligation scheme. Therefore, changes in the axial ligation scheme are predicted to induce large changes in the electron transmission property.
The different transmission functions calculated by employing the WBL approximation for representing the bulk (a) and a first-principles TB model for the bulk represntation (b) at the different spin coupling states of the iron-porphyrin system (4 ligation scheme) are illustrated in the figure. The conductance includes different contributions from the spin α and β channels for the excited spin coupling states.
The considered change in the axial ligation from a five ligated system to a six ligated system, where an addition of a CO ligand as the second axial ligand is illustrated.
Porphyrin orbital diagram for the free molecule and FeP at the different spin coupling states with the gold tip functionalization. Also included in the first two columns are respectively the non thiol functionalized porphyrin and singlet FeP orbitals. Of high interest is the apparent changing contribution of the Fe AOs to the eg porphyrin and AuS orbitals. All unlabeled (black lines) states are of Au[+Fe] character. Pure Fe orbitals have been omitted.}
2. Hydrogen fuel economy
We also pursue additional research venues. In a computational research effort we are engaged in studying systems related to hydrogen adsorption or reaction involving hydrogen production relevent for hydrogen fuel economy: Hydrogen uptake in metal organic frameworks from first principles calculations. Computational models of MOFs have been used to elaborate the hydrogen organic adsorption sites. These calculations are also used to provide suggestions for the improvement of the uptake properties of the MOFs. Further study of molecular models is underway to suggest improved materials for adsorption purposes and to gain insight on the experimental observations. Hydrogen generating reactions. We have collaborated with Sacks group on a study focusing on fragmentation reaction of protonated aromatic systems. These reaction routes can be directed to involve hydrogen generation.
3. Inorganic reaction mechanism
We have also been engaged in studying several reaction mechanisms involving metal centers. This has been pursued in collaboration with the experimental groups executing the synthesis reaction. With collaboration with the Johnson group the reaction mechanism leading to the formation of an unique Ru-carbide has been explored. We have also studied different scheme to functionalize this group. Another system is the cross metathesis of a tungsten center including a nitrile and carbide groups.
4. Excited states studies
We are also considering studies of excited states of extended systems. The method development effort in the area of molecular conductance can also be utilized to study excited states of extended systems as of surface adsorbates (on metallic surfaces as a special difficult case) or the coupling of phonon to electronically excited states. We are also interested in studying excited state of system of biological importance. One example where progress has been achieved is the Co-corrin system within the B12 system.