Session B35: General Theory

The evolution of dielectric properties of sodium, silicon and argon clusters

We used a computational scheme based on site-specific polarizabilities to study the evolution of the dielectric properties of sodium, silicon and argon clusters. In this approach, the total cluster polarizability is decomposed into local dipole (LD) and charge-transfer (CT) parts. The local dipole part measures the redistribution of charge within an atomic volume, while the CT part describes the movement of charge between volumes. We find distinct differences in the relative contributions of the LD and CT components to the total polarizability as a function of cluster size for the different cluster types and relate this to the development of metallic behavior. The method also directly probes the extent of electrostatic screening of the cluster interior to an applied electric field.   

Topological Solitons between Two Magic Number In-clusters on a Si(100) Surface Break the Even/Odd Symmetry in the Self-Selection of Their Distance

Depositing particles randomly on a 1D lattice is expected to result in an equal number of particle pairs separated by even or odd lattice units. Unexpectedly, the even-odd symmetry is broken in the self-selection of distances between indium magic-number clusters on a Si(100)-2x1 reconstructed surface. Cluster pairs separated by even units are less abundant because they are linked by silicon atomic chains carrying topological solitons, which induce local strain and create localized electronic states with higher energy. Our findings reveal a unique particle-particle interaction mediated by the presence or absence of topological solitons on alternate lattices.   

Determination of ground state structures of selected medium-sized clusters via first-principles global search

Due to the existence of numerous local isomers on the potential energy surface, determining the most stable structures of medium-sized clusters is one of the most challenging tasks in cluster physics. Recent years, we have explored the potential energy surface of medium-sized clusters using first-principles calculations combined with global search methods like genetic algorithm and simulate annealing. The examples on a few selected examples such as B$_{80}$/B$_{101}$, Na$_{1-3}$Si$_{1-11}$, and (WO$_{3})_{2-12}$ clusters will be briefly illustrated. The size-dependent electronic properties of these clusters will also be discussed and compared with available experimental data.   

Novel building blocks for materials by design: Janus particles and other patchy colloids

The emergent assembly of nonisotropically structured colloidal particles can lead to novel materials with requisite optical or mechanical properties. We have developed two models---one that includes detailed interactions between particles and another that coarse-grains the interactions---so as to explore the equilibrium and dynamics effected by varying interaction heterogeneities. In particular, we have performed a series of simulations of systems consisting of Janus particles---in which each of two hemispheres can be characterized by a single interaction type such as charge or degree of hydrophobicity. The equilibrium structure of Janus clusters has been the subject of experimental and theoretical studies by Grannick and coworkers. We find that the bulk Janus systems give rise to surprising equilibrium structure and dynamics which can be tuned through both the volume fraction and the interactions. The coarse-grained model provides surprisingly good agreement with the more detailed particle-model for the equilibrium structure while overestimating the relaxation rates.   

Structure and Diffusion of Nanoparticles at the Liquid-Vapor Interface

Large-scale molecular dynamics has been used to simulate a layer of nanoparticles diffusing on the surface of a liquid. Both a low viscosity liquid, represented by Lennard-Jones monomers, and a high viscosity liquid, represented by linear homopolymers, are studied. The organization and diffusion of the nanoparticles are studied as the coverage and the contact angle between the nanoparticles and liquid are varied. Results are compared to simulations of identical nanoparticles in two-dimensions. We show that when the interaction between the nanoparticles and liquid is reduced the contact angle increases and the nanoparticles ride higher on the liquid surface, which enables them to diffuse faster. In this case the short range order is also reduced as seen in the pair correlation function. For low contact angles, nanoparticles diffuse into the liquid for high coverages.   

Supersymmetric Quantum Mechanics For Atomic Electronic Systems

We employ our new approach to non-relativistic supersymmetric quantum mechanics (SUSY-QM), (J. Phys. Chem. A 114, 8202(2010)) for any number of dimensions and distinguishable particles, to treat the hydrogen atom in full three-dimensional detail. In contrast to the standard one-dimensional radial equation SUSY-QM treatment of the hydrogen atom, where the superpotential is a scalar, in a full three-dimensional treatment, it is a vector which is independent of the angular momentum quantum number. The original scalar Schr\"odinger Hamiltonian operator is factored into vector ``charge'' operators: $\vec Q$ and $\vec Q^{\dagger}$. Using these operators, the first sector Hamiltonian is written as $H_1 = \vec Q^{\dagger}\cdot \vec Q + E_{0}^1$. The second sector Hamiltonian is a tensor given by $H_{2} = \vec{Q}\vec{ Q}^{\dagger} + E_{0}^11$ and is isospectral with $H_1$. The second sector ground state, $\vec\psi_{0}^{(2)}$, can be used to obtain the excited state wave functions of the first sector by application of the adjoint charge operator. We then adapt the aufbau principle to show this approach can be applied to treat the helium atom.   

Ion correlations in the electrical double layer near liquid/liquid interfaces

Equilibrium properties of the electrical double layer of monovalent salts near the interface between two immiscible electrolyte solutions are studied via Monte Carlo simulations, for several electrolyte concentrations. The corresponding results are collated with experimental data and the classical Verwey-Niessen theory of point ions. The observed differences between the simulation data and the mean field approach of ``back-to-back" electrical double layers stresses the importance of including properly ionic correlations near dielectric discontinuities.   

Molecular Dynamics Simulation of Adsorption of Methane and Chloromethane on Molybdenum via Hybrid EAM/OPLS Interactions

The question of liquid adsorption on the surface of a solid substrate has been of major interest to computational science even from its inception. Accurate depictions of adsorption phenomena require accurate modeling of both phases of the simulated system. In cases in which accuracy of the models leads to the adoption of potentially inconvenient or incompatible force fields, questions arise as to whether joint systems can perform as well as needed to gather optimal data. The current investigation uses molecular dynamics (MD) tools to focus, in part, on how well-developed models for liquid simulation (OPLS, built on a Lennard-Jones foundation) can combine with tested models for metals (based on functionals of electron density) to describe the adsorption of methane and chloromethane on the surface of molybdenum. Comparisons have been made between the differences in effect of a hybrid EAM/OPLS system and systems focusing solely on Lennard-Jones-based interactions, as well as the effect of large asymmetry and polarity differences in adsorbate species, on the structure and dynamics of layers adsorbed directly at the surface of the metal substrate. Preliminary results suggest that the surface landscape and corrugation play a significant role in choice of surface binding sites.   

First Principles Study of Adsorption of $O_{2}$ on Al Surface with Hybrid Functionals

Adsorption of $O_{2}$ molecule on Al surface has been a long standing puzzle for the first principles calculation. We have studied the adsorption of $O_{2}$ molecule on the Al(111) surface using hybrid functionals.In contrast to the previous LDA/GGA, the present calculations with hybrid functionals successfully predict that $O_{2}$ molecule can be absorbed on the Al(111) surface with a barrier around 0.2$\sim$0.4 eV , which is in good agreement with experiments. Our calculations predict that the LUMO of $O_{2}$ molecule is higher than the Fermi level of the Al(111) surface, which is responsible for the barrier of the $O_{2}$ adsorption.   

Octaselenododecane $(C_4 H_8 Se_{8} )$: a novel polyselenoether crown macrocycle

In this work we have used density-functional theory (DFT/GGA-PBE) to calculate the structural, electronic, and vibrational properties of octaselenododecane $(C_4 H_8 Se_{8} )$, a novel twelve-membered crown-shaped heterocycle which contains four diselenide groups.\footnote {G. Hua, J. M. Griffin, S. E. Ashbrook, A. M. Z. Slawin, and J. D. Wollins, {\it Angew Che. Int. Ed.} {\bf 2011,} 50, 4123-4126.} Our all-electron DFT calculations have yielded results that are in excellent agreement with the observed experimental x-ray diffraction data and infrared and Raman vibrational spectra for the solid state phase of octaselenododecane. In addition to obtaining good general agreement with the selected IR and Raman frequencies reported to lie within the range of 282-2925 cm$^{-1}$, we have obtained other vibrational modes which have not been reported in the literature. In particular, we have computed a Raman active mode at 267 cm$^{-1}$ which is in good agreement with the experimental band at 282 cm$^{-1}$ and have determined that it represents significant asynchronous stretches of diselenide groups within the heterocycle. Our gas phase calculations also show the presence of strong low frequency distortions that are supressed in the crystal due to close Se-Se intramolecular interactions.   

Bias Dependent Switching Behavior of STM-Induced Melamine/Cu(001) Switch

The manipulation or stimulation of molecules using Scanning Tunneling Microscopy (STM) is a technique that recently has deserved deep attention for its potential applications in molecular electronics. The melamine/Cu(001) system was found to show switching behavior in very wide range of applied bias. Although its mechanism was analyzed by a statistic model, the relationship between the switching rate and bias is still far to be fully clarified. In this context, we performed a campaign of exhaustive first-principles calculations to obtain most of the parameters for resonance model; such model is able to predict the switching rate as functions of bias and current. The energy barrier was calculated using the nudged elastic band method, with the aid of recent implementation of current-induced forces into SMEAGOL code, which is based on the nonequilibrium Green's function method with Density Functional Theory. The electron-phonon coupling and then the Inelastic Tunneling Spectroscopy signal are calculated to validate the one-phonon approximation. The spatial distribution of molecular orbitals and their coupling with vibrational modes are very useful to understand the switching behavior.   

Kirkwood-Buff analysis of liquid mixtures in AdResS: Towards an open boundary simulation scheme

Many biophysical processes in water are determined by interactions of cosolvents with the hydration shells of dissolved (bio)molecules. Computational approaches to study these systems are mostly limited to the closed boundary simulations. While closed boundaries are perfectly suitable in many cases, problems arise when concentration fluctuations are large, or intimately linked to the physical phenomenon. For example, in non-ideal mixtures of water/cosolvent and a biomolecule, the excess of water/cosolvent, close to a protein surface, leads to water/cosolvent depletion elsewhere. This complicates a comparison with experiments that are conducted under osmotic conditions. Therefore, we use Adaptive Resolution Simulation (AdResS) scheme, which describes a small sub-volume of a much larger system in atomistic detail, maintaining thermodynamic equilibrium with a surrounding coarse grained reservoir. We show that the Kirkwood-Buff integrals (KBI), which directly connect thermodynamic properties to the molecular distributions, can be efficiently calculated within the small open boundary all atom region and the coarse-grained reservoir maintains the correct particle fluctuations. Results will be presented for the methanol/water mixture and solvation of amino acids in urea/water mixture.