Konstantin N. Kudin

Bending Properties of Single Functionalized Graphene Sheets Probed by Atomic Force Microscopy

We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.

Atomic orbital Laplace-transformed second-order Møller–Plesset theory for periodic systems

We present an atomic-orbital formulation of second-order Moller–Plesset (MP2) theory for periodic systems. Our formulation is shown to have several advantages over the conventional crystalline orbital formulation. Notably, the inherent spatial decay properties of the density matrix and the atomic orbital basis are exploited to reduce computational cost and scaling. The multidimensional k-space integration is replaced by independent Fourier transforms of weighted density matrices. The computational cost of the correlation correction becomes independent of the number of k-points used. Focusing on the MP2 quasiparticle energy band gap, we also show using an isolated fragment model that the long range gap contributions decay rapidly as 1/R5, proof that band gap corrections converge rapidly with respect to lattice summation. The correlated amplitudes in the atomic orbital (AO) basis are obtained in a closed-form fashion, compatible with a semidirect algorithm, thanks to the Laplace transform of the energy deno...

A black-box self-consistent field convergence algorithm: One step closer

A direct inversion iterative subspace version of the relaxed constrained algorithm is found to be a very powerful convergence acceleration technique for the solution of the self-consistent field equations found in the Hartree–Fock method and Kohn–Sham-based density functional theory (KS-DFT). The present algorithm, abbreviated EDIIS, is benchmarked against the direct inversion iterative subspace method based on the commutator of the density and Fock matrices developed by Pulay (DIIS). Our findings indicate that while EDIIS is able to rapidly bring the density matrix from any initial guess to a solution region, the DIIS method is faster when the density matrix is close to convergence. Consequently, we propose a combination of EDIIS and DIIS methods, which is both very robust and highly efficient. We also show how EDIIS can detect the presence and determine the value of fractional occupations in KS-DFT.

Why Are Water−Hydrophobic Interfaces Charged?

We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.

A fast multipole algorithm for the efficient treatment of the Coulomb problem in electronic structure calculations of periodic systems with Gaussian orbitals

Abstract An efficient implementation of the Gaussian very fast multipole method (GvFMM) for periodic systems (pGvFMM) is presented. Relevant details of our algorithm are discussed and linear scaling properties with unit cell size demonstrated on benchmarks using minimum and double-zeta bases on solid NaCl, (5,5) and (10,10) carbon nanotubes containing up to 960 atoms in the unit cell.

Zigzag graphene nanoribbons with saturated edges.

Zigzag graphene nanoribbons with saturated edges are investigated by first principles calculations. In these structures edge carbons have either two H or two F atoms, and are of sp(3) type. Compared to the previously studied ribbons with all carbons of sp(2) type, several similarities and differences are found. Specifically, in narrower ribbons the closed shell electronic state is the most stable one. In wider ribbons a state with antiferromagnetically spin-polarized edges is the lowest in energy, similarly to the ribbons with all sp(2) type carbons. A notable feature of narrower ribbons is significant single-double carbon bond alternation across the ribbon. Calculated Raman spectra contain a distinct blue shift signature of such alternation, which perhaps can be used for the experimental identification of ribbons of this type.

Lattice defects and magnetic ordering in plutonium oxides: A hybrid density-functional-theory study of strongly correlated materials

Experimental studies of actinide oxides are challenging, and conventional electronic structure calculations fail to qualitatively reproduce the scarce data. We employ a new generation of hybrid density functionals to model a defective plutonium dioxide lattice. The procedure is first tested against stoichiometric bulk PuO2 and Pu2O3, for which predictions agree well with experiment where known. The interstitial oxygen in PuO2.25 is found to be singly charged, consistent with experimental observations and contrary to the O2- previously proposed theoretically.

A fast multipole method for periodic systems with arbitrary unit cell geometries

Abstract The point-charge fast multipole method (FMM) for periodic boundary conditions is generalized from cubic to rectangular simulation cells. This development let us treat lattices with orthogonal (rectangular) unit cells. The lattice of non-orthogonal systems can be transformed to yield a rectangular simulation cell. Thus, our periodic FMM algorithm can be applied to lattices with arbitrary unit cells. We also discuss in detail our proposed solutions for problems arising from the accuracy of the infinite summation contribution and the dipole moment of the simulation cell. Benchmark results of our periodic FMM show linear-scaling properties.

Electronic Response Properties of Carbon Nanotubes in Magnetic Fields

Magnetic linear response properties for achiral and chiral carbon nanotubes were investigated with first-principles electronic structure methods. We have computed the magnetic shielding inside and outside the tubes originating from electronic current densities induced by the application of an external magnetic field. This electronic response of the nanotubes was analyzed for external magnetic fields both parallel and perpendicular to the tube axis. The magnetic screening would be experienced by guest molecules inside the tubes, measurable by NMR spectroscopy on isotopically labeled samples. Special attention is given to chiral nanotubes, in which longitudinal fields induce a non-zero longitudinal current density and thus tangential magnetic fields outside the tubes.

Single wall carbon nanotubes density of states: comparison of experiment and theory

Abstract We study the electronic structure of a variety of single wall carbon nanotubes and report density of states obtained with the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation and hybrid PBE0 approximation of density functional theory using Gaussian orbitals and periodic boundary conditions. PBE gives very good results for metallic tubes but the addition of a portion of exact exchange in the hybrid PBE0 functional worsens the agreement between experiment and theory. On the other hand, the PBE0 hybrid significantly improves the theoretical predictions (compared to PBE) for semiconducting tubes.