Mark R. Pederson

Local‐density Hartree–Fock theory of electronic states of molecules with self‐interaction correction

A scheme for incorporating the self‐interaction correction (SIC) to the local density approximation of the Hartree–Fock theory of electronic structure of molecules is presented. This method is applied to the N2 molecule and the resulting orbital energies and total energy are in good agreement with the Hartree–Fock values.

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.

Strategies for Massively Parallel Local‐Orbital‐Based Electronic Structure Methods

We discuss several aspects related to massively parallel electronic structure calculations using the gaussian-orbital based Naval Research Laboratory Molecular Orbital Library (NRLMOL). While much of the discussion is specific to gaussian-orbital methods, we show that all of the computationally intensive problems encountered in this code are special cases of a general class of problems which allow for the generation of parallel code that is automatically dynamically load balanced. We refer to the algorithms for parallelizing such problems as “honey-bee algorithms” because they are analogous to natures way of generating honey. With the use of such algorithms, BEOWULF clusters of personal computers are roughly equivalent to higher performance systems on a per processor basis. Further, we show that these algorithms are compatible with more complicated parallel programming architectures that are reasonable to anticipate. After specifically discussing several parallel algorithms, we discuss applications of this program to magnetic molecules.

First principles determination of the interatomic force-constant tensor of the fullerene molecule

Abstract We have determined the full force constant tensor (Hessian) of the fullerene molecule, C 60 from first principles. From our all-electron density functional code, the forces on all of the atoms are calculated for different displacements of a single atom. Using finite differences and the symmetry operations of the molecule, the full matrix is determined. Diagonalization of the dynamical matrix yields the vibrational modes, which are in excellent agreement with experiment. The range and nature of the force constant tensor will be presented.

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.

Communication: Self-interaction correction with unitary invariance in density functional theory

Standard spin-density functionals for the exchange-correlation energy of a many-electron ground state make serious self-interaction errors which can be corrected by the Perdew-Zunger self-interaction correction (SIC). We propose a size-extensive construction of SIC orbitals which, unlike earlier constructions, makes SIC computationally efficient, and a true spin-density functional. The SIC orbitals are constructed from a unitary transformation that is explicitly dependent on the non-interacting one-particle density matrix. When this SIC is applied to the local spin-density approximation, improvements are found for the atomization energies of molecules.

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.

Hamiltonian of the V15 spin system from first-principles density-functional calculations.

We report first-principles all-electron density-functional-based studies of the electronic structure, magnetic ordering, and anisotropy for the V15 molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with five antiferromagnetic and one ferromagnetic exchange couplings. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin S = 1/2 ground state and low-lying S = 3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.

Localized and canonical atomic orbitals in self‐interaction corrected local density functional approximation

The self‐interaction corrected (SIC) version of the local spin density (LSD) approximation has been applied to the first two rows of the periodic table with particular emphasis on local orbital choice. These are the first SIC–LSD calculations for atomic systems that account for all nonspherical corrections and are based on a rigorous variational theory. The resulting total energies and orbital energies are improved in comparison to experiment and restore a desirable trend which is found in Hartree–Fock theory. We demonstrate that with a proper treatment of the SIC–LSD off‐diagonal Lagrange multipliers, the viral theorem is satisfied at self‐consistency.