Tunna Baruah

A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers.

We report first-principles density-functional calculations for hydroquinone (HQ), indolequinone (IQ), and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of biomacromolecules with important biological functions (including photoprotection) and with the potential for certain bioengineering applications. We have used the difference of self-consistent fields method to study the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, Delta(HL). We show that Delta(HL) is similar in IQ and SQ, but approximately twice as large in HQ. This may have important implications for our understanding of the observed broadband optical absorption of the eumelanins. The possibility of using this difference in Delta(HL) to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to nondestructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behavior of the eumelanins.

Density functional studies of single molecule magnets

Abstract A method for the calculation of the second-order anisotropy parameters of single molecular magnets from the single particle orbitals is reviewed. We combine this method with density functional calculations to predict the magnetic anisotropy parameters of several single molecule magnets: Mn 12 -acetate, Mn 10 , Co 4 , Fe 4 , Cr 1 and V 15 . Comparison with available experimental data shows that it is possible to predict these values quite accurately from density functional wavefunctions.

Kondo Resonances and Anomalous Gate Dependence in the Electrical Conductivity of Single-Molecule Transistors

We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.

DFT Calculations on Charge-Transfer States of a Carotenoid-Porphyrin-C60 Molecular Triad.

We present a first-principles study on the ground and excited electronic states of a carotenoid-porphyrin-C60 molecular triad. In addition, we illustrate a method for using DFT-based wave functions and densities to simulate complicated charge-transfer dynamics. Since fast and efficient calculations of charge-transfer excitations are required to understand these systems, we introduce a simple DFT-based method for calculating total energy differences between ground and excited states. To justify the procedure, we argue that some charge-transfer excitations are asympototically ground-state properties of the separated systems. Further justification is provided from numerical experiments on separated alkali atoms. The donor-chromophore-acceptor system studied here can absorb and store light energy for several hundreds of nanoseconds. Our density-functional calculations show that the triad can possess a dipole moment of 171 D in a charge-separated state. The charge-transfer energy technique is used to obtain the energies of the excited states. The charge separated excited states with a large dipole moment will create large polarization of the solvent. We use a model to estimate the stabilization of the excited-state energies in the presence of polarization. The calculated excited-state energies are further used to calculate the Einsteins A and B coefficients for this molecular system. We use these transition rates in a kinetic Monte-Carlo simulation to examine the electronic excitations and possible charging of the molecule. Our calculations show that the solvent polarization plays a crucial role in reordering the excited-state energies and thereby in the charge-separation process.

Static dielectric response of icosahedral fullerenes from C60 to C2160 characterized by an all-electron density functional theory

The static dielectric response of C60, C180, C240, C540, C720, C960, C1500, and C2160 fullerenes is characterized by an all-electron density-functional method. First, the screened polarizabilities of C60, C180, C240, and C540, are determined by the finite-field method using Gaussian basis set containing 35 basis functions per atom. In the second set of calculations, the unscreened polarizabilities are calculated for fullerenes C60 through C2160 from the self-consistent Kohn-Sham orbitals and eigen-values using the sum-over-states method. The approximate screened polarizabilities, obtained by applying a correction determined within linear response theory show excellent agreement with the finite-field polarizabilities. The static dipole polarizability per atom in C2160 is (4 Angstrom^3) three times larger than that in C60 (1.344 Angstrom^3). Our results reduce the uncertainty in various theoretical models used previously to describe the dielectric response of fullerenes and show that quantum size effects in polarizability are significantly smaller than previously thought.

Density functional study on a light-harvesting carotenoid -porphyrin -C60 molecular triad

We present a study on the electronic structure of a biology-inspired molecular triad which shows promises in replicating photosynthesis process in the laboratory. The triad contains three different units--C60, porphyrin, and beta-carotenoid. We present its geometrical and electronic structure, dipole moments, optical absorption spectrum, and polarizability calculated with an all-electron density functional approach. Such a study will be useful for further understanding of its photoconversion properties.

Vibrational stability and electronic structure of a B80 fullerene

We investigate the vibrational stability and the electronic structure of the proposed icosahedral fullerenelike cage structure of

Tuning Molecule-Mediated Spin Coupling in Bottom-Up-Fabricated Vanadium-Tetracyanoethylene Nanostructures

{\text{B}}_{80}

Density-functional study of two Fe4-based single-molecule magnets

[N. G. Szwacki, A. Sadrzadeh, and B. I. Yakobson, Phys. Rev. Lett. 98, 166804 (2007)], by an all electron density-functional theory using polarized Gaussian basis functions containing 41 basis functions per atom. The vibrational analysis of

Charge transfer excited state energies by perturbative delta self consistent field method

{\text{B}}_{80}