Search

In Brownian Dynamics simulations, the diffusive motion of the particles is simulated by adding random displacements, proportional to the square root of the chosen time step. When computing average quantities, these Brownian contributions usually average out, and the overall simulation error becomes proportional to the time step. A special situation arises if the particles undergo hard-body interactions that instantaneously change their properties, as in absorption or association processes, chemical reactions, etc. The common "naive simulation method" accounts for these interactions by checking for hard-body overlaps after every time step. Due to the simplification of the diffusive motion, a substantial part of the actual hard-body interactions is not detected by this method, resulting in an overall simulation error proportional to the square root of the time step. In this paper we take the hard-body interactions during the time step interval into account, using the relative positions of the particles at the beginning and at the end of the time step, as provided by the naive method, and the analytical solution for the diffusion of a point particle around an absorbing sphere. Ottinger used a similar approach for the one-dimensional case [Stochastic Processes in Polymeric Fluids (Springer, Berlin, 1996), p. 270]. We applied the "corrected simulation method" to the case of a simple, second-order chemical reaction. The results agree with recent theoretical predictions [K. Hyojoon and Joe S. Kook, Phys. Rev. E 61, 3426 (2000)]. The obtained simulation error is proportional to the time step, instead of its square root. The new method needs substantially less simulation time to obtain the same accuracy. Finally, we briefly discuss a straightforward way to extend the method for simulations of systems with additional (deterministic) forces. (C) 2002 American Institute of Physics.

In molecular J-aggregates one often observes an increase of the fluorescence decay time when increasing the temperature from 0 K. This phenomenon is usually attributed to the thermal population of the dark Frenkel exciton states that lie above the superradiant bottom state of the exciton band. In this paper, we study this effect for a homogeneous one-dimensional aggregate in a host medium and we model the scattering between different exciton states as arising from their coupling to the host vibrations. A Pauli master equation is used to describe the redistribution of excitons over the band. The rates entering this equation are calculated within the framework of first-order perturbation theory, assuming a linear on-site interaction between excitons and acoustic phonons. Solving the master equation numerically for aggregates of up to 100 molecules, we calculate the temperature dependence of the fluorescence kinetics in general and the decay time scale in particular. The proper definition of the fluorescence decay time is discussed in detail. We demonstrate that, even at a quantum yield of unity, the possibility to directly interpret fluorescence experiments in terms of a simple radiative time scale depends crucially on the initial excitation conditions in combination with the competition between spontaneous emission and intraband phonon-assisted relaxation.

In this paper we show how relativistic effects can be included in the time-dependent density-functional theory (DFT) for the optical response properties of nonmetallic crystals. The dominant scalar relativistic effects have been included using the zeroth-order regular approximation (ZORA) in the ground-state DFT calculations, as well as in the time-dependent response calculations. We show that this theory can also be applied to indium antimonide in the zinc-blende structure, notwithstanding the fact that it turns into a semimetal when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant ε∞ and the dielectric function ε(ω), for the various levels on which relativity can be included, i.e., nonrelativistic, only in the ground-state, or also in the response calculation. Comparisons of our calculated results are made with experiment and other theoretical investigations. With the inclusion of scalar relativistic effects, the band structure of InSb becomes semimetallic within the local density approximation and we find a deviation of 5% from experiment for the static dielectric constant. Also the dielectric function is improved and the spectra are in good agreement with experiment, although the spectral features are shifted to somewhat lower energies compared to experiment.

In this paper we present the implementation of the two-component scaled zeroth-order regular approximation (ZORA) method in the molecular electronic structure package GAMESS-UK. It is the first application of this method, which was earlier investigated in the context of density functional theory, in molecular ab initio basis set calculations. The performance of the method is tested in atomic calculations, which we can compare with numerical results, on xenon and radon and in molecular calculations on the molecules AgH, HI, I2, AuH, TlH, and Bi2. In calculations on the I2 molecule we investigated the effect of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed harmonic frequencies and bond lengths. It is shown that the scaled ZORA approach is a cost-effective alternative to the Dirac–Fock method.

The key characteristics of electronic excitations of many-electron systems, the excitation energies ωα and the oscillator strengths fα, can be obtained from linear response theory. In one-electron models and within the adiabatic approximation, the zeros of the inverse response matrix, which occur at the excitation energies, can be obtained from a simple diagonalization. Particular cases are the eigenvalue equations of time-dependent density functional theory (TDDFT), time-dependent density matrix functional theory, and the recently developed phase-including natural orbital (PINO) functional theory. In this paper, an expression for the oscillator strengths fα of the electronic excitations is derived within adiabatic response PINO theory. The fα are expressed through the eigenvectors of the PINO inverse response matrix and the dipole integrals. They are calculated with the phaseincluding natural orbital functional for two-electron systems adapted from the work of Lowdin ¨ and Shull on two-electron systems (the phase-including Löwdin-Shull functional). The PINO calculations reproduce the reference fα values for all considered excitations and bond distances R of the prototype molecules H2 and HeH+ very well (perfectly, if the correct choice of the phases in the functional is made). Remarkably, the quality is still very good when the response matrices are severely restricted to almost TDDFT size, i.e., involving in addition to the occupied-virtual orbital pairs just (HOMO+1)-virtual pairs (R1) and possibly (HOMO+2)-virtual pairs (R2). The shape of the curves fα(R) is rationalized with a decomposition analysis of the transition dipole moments. peerReviewed

In this paper a detailed theoretical analysis is presented of the temperature dependent radiative decay in aggregates of pseudoisocyanine (PIC). Our approach extends the original linear exciton-phonon coupling model including static disorder and second order exciton-phonon interactions. It is shown that for a one-dimensional exciton model neither of these additional effects alone or in combination with linear electron–phonon coupling can explain the steep rise in radiative lifetime at 40 K observed in the J-aggregate of PIC. However, when the aggregate assembles into a two-dimensional bricklike structure its radiative dynamics can be simulated, with linear exciton-optical phonon coupling as the only source for exciton scattering. Exciton-phonon scattering transfers oscillator strength from the k=0 state to other band states and also generates a nonequilibrium population among the exciton states, which persists during the superradiant decay. These effects together explain the marked temperature dependence of the radiative lifetime of the PIC J aggregate. When disorder limits the coherence length at low temperatures to a few molecules, as seems the case in several light harvesting complexes, the exciton population can equilibrate on the time scale of the superradiance. This situation pertains to the strong collision limit of the master equation, where the radiative decay is insensitive to details of the electron–phonon coupling, but only senses change in the thermal population among the exciton states.

The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm−1 by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K⎯=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

The discrete Kohn variational reactive scattering method presented in the preceding paper is applied to the reaction H+O2→ OH+O. The essential features of the method are the use of an interaction region grid, fully coupled open and closed distorted waves (DWs) and the use of a singular value decomposition technique to construct the optimal boundary conditions for the open DWs. A convergence test is presented at a total energy of 0.817 eV above the bottom of the H+O2 well. It is shown that very well converged results may be obtained in a calculation with a relatively small interaction region grid, when at least a few asymptotically closed DWs are included in the trial wave function. Furthermore, the number of open distorted waves (DWs) may be considerably smaller than the number of open channels. Six additional points are computed in an energy range of 1.2 meV, scanning through a narrow resonance around 0.817 eV. The results are in very good agreement with the hyperspherical coordinate propagation calculat...

In this paper we present time-dependent density functional calculations on frequency-dependent first (β) and second (γ) hyperpolarizabilities for the set of small molecules, N2, CO2, CS2, C2H4, NH3, CO, HF, H2O, and CH4, and compare them to Hartree–Fock and correlated ab initio calculations, as well as to experimental results. Both the static hyperpolarizabilities and the frequency dispersion are studied. Three approximations to the exchange-correlation (xc) potential are used: the widely used Local Density Approximation (LDA), the Becke–Lee–Yang–Parr (BLYP) Generalized Gradient Approximation (GGA), as well as the asymptotically correct Van Leeuwen–Baerends (LB94) potential. For the functional derivatives of the xc potential the Adiabatic Local Density Approximation (ALDA) is used. We have attempted to estimate the intrinsic quality of these methods by using large basis sets, augmented with several diffuse functions, yielding good agreement with recent numerical static LDA results. Contrary to claims which have appeared in the literature on the basis of smaller studies involving basis sets of lesser quality, we find that the static LDA results for β and γ are severely overestimated, and do not improve upon the (underestimated) Hartree–Fock results. No improvement is provided by the BLYP potential which suffers from the same incorrect asymptotic behavior as the LDA potential. The results are however clearly improved upon by the LB94 potential, which leads to underestimated results, slightly improving the Hartree–Fock results. The LDA and BLYP potentials overestimate the frequency dependence as well, which is once again improved by the LB94 potential. Future improvements are expected to come from improved models for asymptotically correct exchange-correlation potentials. Apart from the LB94 potential used in this work, several other asymptotically correct potentials have recently been suggested in the literature and can also be expected to improve considerably upon the relatively poor LDA and GGA results, for both the static properties and their frequency dependence.

In the present paper we consider nonlinear elasticity and friction of grafted persistent chains, which are highly stretched in the normal to the surface direction due to orientational interactions. We examine the normal and the lateral forces both in equilibrium and under shear sliding when the monolayer is confined by a bare surface. We show that in the confined monolayer in equilibrium the tilted orientation of the director becomes stable. In the sliding regime the friction force passes through a maximum value. The additional normal force in the sliding regime, when the distance between the surfaces is fixed, is also considered. We show that this force is attractive for small velocities and becomes repulsive for high velocities after the friction force passes through the maximum value.

In this paper, we study the evolution of phase-separating binary mixtures which are subjected to alternate cooling and heating cycles. An initially homogeneous mixture is rapidly quenched to a temperature T(1)T(c), where T(c) is the critical temperature. The mixture undergoes phase separation for a while and is then suddenly heated to a temperature T(2)T(c). These cycles are repeated to create a domain morphology with multiple length scales, i.e., the structure factor is characterized by multiple peaks. For phase separation in d = 2 systems, we present numerical and analytical results for the emergence and growth of this multiple-scale morphology.

In this paper we apply nonlocal kinetic coupling to the dynamic mean-field density functional method, which is derived from generalized time-dependent Ginzburg-Landau theory. The method is applied to the mesoscopic dynamics of copolymer melts, which was previously simulated using a local coupling approximation. We discuss the general theory of time evolution of density variables with general kinetic coefficients developed by Kawasaki and Sekimoto, and especially the limits of the theory that yield the local coupling approximation, the collective Rouse dynamics model, and the reptation dynamics model. We show bow a simple approximation to the Rouse dynamics model leads to a feasible numerical model that includes the essential physical features of nonlocal kinetic coupling. This results in a dynamic equation for the external potential instead of the density which allows us to perform calculations of microphase separation in copolymer melts with increased relevance to experimental results. As may be expected from a numerical model that includes nonlocal kinetic coupling, the numerical results show an increased computational efficiency, less defects in the final morphology, and a faster increase of the order parameter compared to local kinetic coupling. (C) 1997 American Institute of Physics.

In this paper we discuss a new generalized time-dependent Ginzburg-Landau theory for the numerical calculation of polymer phase separation kinetics in 3D. The thermodynamic forces are obtained by a mean-field density functional method, using a Gaussian chain as a molecular model. The method is especially aimed at describing the formation kinetics of the irregular morphologies which are typical for many industrial systems. As proof of concept we present the formation of irregular morphologies in quenched symmetric and asymmetric block copolymer melts. (C) 1997 American Institute of Physics.

The flux-flux correlation function formalism is a standard and widely used approach for the computation of reaction rates. In this paper we introduce a method to compute the classical and quantum flux-flux correlation functions for anharmonic barriers essentially analytically through the use of the classical and quantum normal forms. In the quantum case we show that the quantum normal form reduces the computation of the flux-flux correlation function to that of an effective one dimensional anharmonic barrier. The example of the computation of the quantum flux-flux correlation function for a fourth order anharmonic barrier is worked out in detail, and we present an analytical expression for the quantum mechanical microcanonical flux-flux correlation function. We then give a discussion of the short-time and harmonic limits., 9 pages, 2 figures

Low-frequency intermolecular dynamics in liquids is studied by ultrafast four- and six-wave mixing. The theory of these nonlinear optical processes is given for electronically nonresonant optical interactions up to fifth order in the electric field. The Born-Oppenheimer approximation is used to separate the motional part of the response functions from coordinate independent electronic hyperpolarizabilities. A large variety of experiments, involving far-infrared absorption, ordinary Rayleigh-Raman or hyper Rayleigh-Raman scattering is covered by this theory. The response in nonresonant six-wave mixing comprises four dynamically different processes. It is shown that one of the terms contains information on the time scale(s) of intermolecular dynamics, that is not available from lower-order nonresonant experiments. For instance, homogenous and inhomogeneous contributions to line broadening can be distinguished. The optical response of harmonic nuclear motion is calculated for nonlinear coordinate dependence of the polarizabilities. Results for level-dependent and level-independent damping of the motion are compared. It is shown that level-dependent damping destroys the interference between different quantum mechanical pathways, yielding an extra contribution to the fifth-order response that has not been discussed before. When two or more nuclear modes determine the optical response, their relative contributions to the four- and six-wave mixing signals are in general different. These contributions are determined by the coordinate dependence of the electronic polarizability, which is usually not fully known, Model calculations are presented for the dynamic parameters of liquid CS2. The theory of this paper will be employed in Part II, to analyze experimental results on femtosecond four- and six-wave mixing.

The exchange-correlation potentials v xc which are currently fashionable in density functional theory ~DFT!, such as those obtained from the local density approximation ~LDA! or generalized gradient approximations ~GGAs!, all suffer from incorrect asymptotic behavior. In atomic calculations, this leads to substantial overestimations of both the static polarizability and the frequency dependence of this property. In the present paper, it is shown that the errors in atomic static dipole and quadrupole polarizabilities are reduced by almost an order of magnitude, if a recently proposed model potential with correct Coulombic long-range behavior is used. The frequency dependence is improved similarly. The model potential also removes the overestimation in molecular polarizabilities, leading to slight improvements for average molecular polarizabilities and their frequency dependence. For the polarizability anisotropy we find that the model potential results do not improve over the LDA and GGA results. Our method for calculating frequency-dependent molecular response properties within time-dependent DFT, which we described in more detail elsewhere, is summarized. © 1996 American Institute of Physics.@S0021-9606~96!02031-4#

In this paper, an extensive analysis of free volume related properties of a lipid membrane is given. Using percolation theory, and comparing the free volume properties to those of a soft polymer, additional insights are obtained. The analyses are discussed within the framework of the four region model. It is concluded that the four regions have very different free volume properties. The region containing the dense part of the lipid tails resembles a soft polymer membrane to a large extent. The middle part of the membrane is more similar to a low density alkane. The consequences of the computed free volume properties on the permeation process of small penetrants are discussed. (C) 1996 American Institute of physics.

A new theory is presented for calculating the Helmholtz free energy based on the potential energy distribution function. The usual expressions of free energy, internal energy and entropy involving the partition function are rephrased in terms of the potential energy distribution function, which must be a near Gaussian one, according to the central limit theorem. We obtained expressions for the free energy and entropy with respect to the ideal gas, in terms of the potential energy moments. These can be linked to the average potential energy and its derivatives in temperature. Using thermodynamical relationships we also produce a general differential equation for the free energy as a function of temperature at fixed volume. In this paper we investigate possible exact and approximated solutions. The method was tested on a theoretical model for a solid (classical harmonic solid) and some experimental liquids. The harmonic solid has an energy distribution, which can be derived exactly from the theory. Experimental free energies of water and methanol could be reproduced very well over a temperature range of more than 300 K. For water, where the appropriate experimental data were available, also the energy and heat capacity could be reproduced very well. (C) 1996 American Institute of Physics.

In quantum-mechanical/molecular-mechanical (QM/MM) treatment of chemical reactions in condensed phases, one solves the electronic Schrodinger equation for the solute (or an active site) under the electrostatic field from the environment. This Schrodinger equation depends parametrically on the solute nuclear coordinates R and the external electrostatic potential V. This fact suggests that one may use R and V as natural collective coordinates for describing the entire system, where V plays the role of collective solvent variables. In this paper such an (R,V) representation of the QM/MM canonical ensemble is described, with particular focus on how to treat charge transfer processes in this representation. As an example, the above method is applied to the proton-coupled electron transfer of a ubiquinol analog with phenoxyl radical in acetonitrile solvent. Ab initio free-energy surfaces are calculated as functions of R and V using the reference interaction site model self-consistent field method, the equilibrium points and the minimum free-energy crossing point are located in the (R,V) space, and then the kinetic isotope effects (KIEs) are evaluated approximately. The results suggest that a stiffer proton potential at the transition state may be responsible for unusual KIEs observed experimentally for related systems.

In this paper, we describe a numerical method for the calculation of collective diffusion relaxation mechanisms in quenched block copolymer melts. The method entails the repeated calculation of two opposing fields-an external potential field U, conjugate to the density field rho, and an energetic interaction field E. The external field is calculated by numerical inversion of the density functionals and the energetic interaction field is calculated directly by integration over the density field. When the two fields are balanced U = E, we recover the self-consistent field solutions; when the two fields are off balance, the spatial gradient of E-U is the thermodynamic force which drives the collective diffusion. We introduce a simple local coupling approximation for the Onsager kinetic coefficients of short freely jointed chains in weakly ordered systems. Fluctuations are added by incorporation of a random Langevin force in the diffusion equation. Numerical results of decomposition in symmetric and asymmetric diblock copolymer melts indicate that the method is capable of describing extremely slow defect annihilation relaxation modes. We find that in the nonlinear regime, the density patterns evolve to metastable states, in which isolated defects separate relatively well-ordered crystalline microdomains. These final states are typical for many industrial applications of incompletely relaxed copolymer melts.

In this paper we present a comprehensive lattice Monte Carlo study of long chain conformations at solid-polymer melt interfaces. Segmental scale interfacial features, like the bond orientational distribution were found to be independent of surface-segment energetics, and statistically identical with Helfand’s predictions for the full-occupancy, infinite chain length limit. Conformational statistics of chains longer than 5-6 statistical segments followed the predictions of the Scheutjens-Fleer theory and the same power laws as a single ideal chain at the critical value of the surface-segment adsorption free-energy. Our simulations tested the predictions of random walk next to a “reflective” surface statistics for the spatial variations of chain dimensions and chain center of mass density. It was found that these statistics furnish the correct long chain limit, independently of surface-segment energetics. The random walk next to a “reflective” boundary predictions for the “adsorbed” amount and the distributions of tail, loop, and train number, as well as tail and loop size were in quantitative agreement with the simulation data. The correspondence between random walks and real (or simulated) chains required the knowledge of a single, microscopic parameter, the number of chemical segments per statistical segment, a∞. This quantity was very close to the average length of adsorbed sequences (trains), in the long chain limit. Our simulations tested thoroughly and established firmly the validity of “reflective” boundary statistics in the melt. The inevitability of these statistics has broad implications on “desorption” kinetics, chain mobility, and chain relaxation, which are currently under study.

In this paper, we report the first rotationally resolved one- and two-color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE-PFI) experiments carried out in a "magnetic bottle'' electron spectrometer. A direct comparison is made between ZEKE-PFI and REMPI-PES spectra for ionization via several rotational levels of the F 12(v'=0) and f 32(v'=0) Rydberg states of HCl. Large differences in both the spin-orbit and rotational branching ratios are found between the ZEKE-PFI and REMPI-PES spectra. These differences can be understood qualitatively on the basis of rotational and spin-orbit autoionization mechanisms. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.

The fifth-order two-dimensional (2D) Raman signals have been calculated from the equilibrium and nonequilibrium (finite field) molecular dynamics simulations. The equilibrium method evaluates response functions with equilibrium trajectories, while the nonequilibrium method calculates a molecular polarizability from nonequilibrium trajectories for different pulse configurations and sequences. In this paper, we introduce an efficient algorithm which hybridizes the existing two methods to avoid the time-consuming calculations of the stability matrices which are inherent in the equilibrium method. Using nonequilibrium trajectories for a single laser excitation, we are able to dramatically simplify the sampling process. With this approach, the 2D Raman signals for liquid xenon, carbon disulfide, water, acetonitrile, and formamide are calculated and discussed. Intensities of 2D Raman signals are also estimated and the peak strength of formamide is found to be only five times smaller than that of carbon disulfide.

The ground state structure for the CCCCCl radical was computed by using symmetry-adapted cluster configuration-interaction (SAC-CI) theory along with density functional theory to overcome the differences raised in the recently published paper [Y. Sumiyoshi et al., Chem. Phys. Lett. 414, 82 (2005)] between the theory and the experiment. SAC-CI results clearly support the earlier experimental conclusion that the radical has the bent ground state structure corresponding to 2Pi symmetry. Contrarily, probably due to spin contamination, mixing of a bent doublet ground state with the quartet components of a linear structure, coupled-cluster singles and doubles (CCSD) calculations were unable to provide reliable results. Results obtained using density functional theory also show that the radical has a bent structure. Some low-lying doublet excited states were also studied using the SAC-CI theory. The energy difference between the ground Pi state and the nearby Sigma state is around 0.2 eV. The excitation energy for the transition with the largest oscillator strength agrees with the strongest absorption peak.

In this paper, a full account of an earlier report [Takahashi, Okuyama, and Kimura, J. Mol. Struct. 249, 47 (1991)] on the cation vibrational spectra of trans and gauche n‐propylbenzene, which were obtained by means of a two‐color (1+1’) multiphoton ionization threshold photoelectron technique is presented. The trans and gauche cations were separately produced by (1+1’) multiphoton ionization resonant through the S1 vibronic levels of n‐propylbenzene in supersonic free jets in which both the trans and gauche isomers exist. From the observed threshold photoelectron spectra, adiabatic ionization energies are determined as Ia(trans)=70 278 cm−1 (8.7134 eV) and Ia(gauche)=70 420 cm−1 (8.7311 eV) with an accuracy of ±8 cm−1 (±1 meV). Furthermore, four benzene ring modes (6b+, 1+, 12+, and 18a+) as well as several low‐frequency torsional and bending modes have been identified which are sensitive to the relative conformations of the n‐propyl group with respect to the benzene ring [(trans)+: 82, 212, and 300 cm−1...

This paper discusses the conformational phase change of an isolated polymer chain which is capable of forming physical bonds with solvent molecules. On the basis of the mean‐field theory of Flory type, we derive a formula for the temperature dependence of the expansion factor of the chain. Our theory takes account of the extra mixing entropy change induced by the solvation. We predict that the physical‐bond formation between polymer and solvent molecules causes a reentrant conformational change between coiled and globular state when temperature is varied. We also show that the polymer chain exhibits a sharp collapse near the Θ temperature as the temperature is increased to the lower critical solution temperature of the polymer solution. This behavior can be interpreted in terms of the breakup of the polymer–solvents complex. The result is compared with the observed coil–globule transition on poly (N‐isopropylacrylalmide) in water.

An examination of the finite system size effects on the calculated interfacial properties of the molten salt KI is described, as an extension of the work described in the preceding paper [J. Chem. Phys. 115, 8603 (2001)]. It is shown that, for system sizes beyond ∼1000 ions, the finite size effects are accounted for by the predictions of capillary wave (CW) theory, as in van der Waals liquids. Hence, there do not appear to be additional long-range effects arising from the Coulombic interactions. Techniques used to avoid truncation effects in the Coulombic and dispersion interactions appear to have eliminated system size dependent artefacts from these sources. The scaling behavior predicted from CW theory is used to predict the bulk surface tension for the chosen interaction potential, which is then compared with experimental values, and very good agreement found. Examination of the interfacial layers show that the local structure remains strongly charge ordered, but the surface is highly roughened by clus...

Item does not contain fulltext Transport properties of molecular junctions are notoriously expensive to calculate with ab initio methods, primarily due to the semi-infinite electrodes. This has led to the introduction of different approximation schemes for the electrodes. For the most popular metals used in experiments, such as gold, the wide-band limit (WBL) is a particularly efficient choice. In this paper, we investigate the performance of different WBL schemes relative to more sophisticated approaches including the fully self-consistent non-equilibrium Green's function method. We find reasonably good agreement between all schemes for systems in which the molecule (and not the metal-molecule interface) dominates the transport properties. Moreover, our implementation of the WBL requires negligible computational effort compared to the ground-state density-functional theory calculation of a molecular junction. We also present a new approximate but efficient scheme for calculating transport with a finite bias. Provided the voltage drop occurs primarily inside the molecule, this method provides results in reasonable agreement with fully self-consistent calculations.

Recently, we reported the formation of crystalline monolayers consisting of octapod-shaped nanocrystals (so-called octapods) that had arranged in a square-lattice geometry through drop deposition and fast evaporation on a substrate [W. Qi, J. de Graaf, F. Qiao, S. Marras, L. Manna, and M. Dijkstra, Nano Lett. 12, 5299 (2012)]10.1021/nl302620j. In this paper we give a more in-depth exposition on the Monte Carlo simulations in a quasi-two-dimensional (quasi-2D) geometry, by which we modelled the experimentally observed crystal structure formation. Using a simulation model for the octapods consisting of four hard interpenetrating spherocylinders, we considered the effect of the pod length-to-diameter ratio on the phase behavior and we constructed the full phase diagram. The methods we applied to establish the nature of the phase transitions between the various phases are discussed in detail. We also considered the possible existence of a Kosterlitz-Thouless-type phase transition between the isotropic liquid and hexagonal rotator phase for certain pod length-to-diameter ratios. Our methods may prove instrumental in guiding future simulation studies of similar anisotropic nanoparticles in confined geometries and monolayers.

The hydrogen storage system LiH + NH(3) ↔ LiNH(2) + H(2) is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of LiNaK. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH(3) by calculating the potential barrier of the H(2) desorption process from the reaction of an M(2)H(2) cluster with an NH(3) molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H(2) dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH(3) before the reaction, while that of the H atom in M(2)H(2) takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M(2)H(2)-NH(3) system and succeeded to reproduce the H(2) desorption from the reaction of Na(2)H(2) with NH(3).

In this paper we examine the phase behavior of the Weeks–Chandler–Andersen (WCA) potential with βε = 40. Crystal nucleation in this model system was recently studied by Kawasaki and Tanaka [Proc. Natl. Acad. Sci. U.S.A. 107, 14036 (2010)]10.1021/pr100656u, who argued that the computed nucleation rates agree well with experiment, a finding that contradicted earlier simulation results. Here we report an extensive numerical study of crystallization in the WCA model, using three totally different techniques (Brownian dynamics, umbrella sampling, and forward flux sampling). We find that all simulations yield essentially the same nucleation rates. However, these rates differ significantly from the values reported by Kawasaki and Tanaka and hence we argue that the huge discrepancy in nucleation rates between simulation and experiment persists. When we map the WCA model onto a hard-sphere system, we find good agreement between the present simulation results and those that had been obtained for hard spheres [L. Filion, M. Hermes, R. Ni, and M. Dijkstra, J. Chem. Phys. 133, 244115 (2010)10.1063/1.3506838; S. Auer and D. Frenkel, Nature 409, 1020 (2001)

A new effective Hamiltonian is presented for the analysis of the high-resolution spectra of open-shell van der Waals complexes containing the O2 molecule. The effects of electron spin are included but the complications of nuclear spin and resultant nuclear spin splitting are neglected. The Hamiltonian is composed of the rotational, centrifugal distortion, and spin-spin interaction terms. The resulting energy levels are divided into two well-separated groups and the pattern is a complicated function of θ (the angle that the O2 molecule makes with the principal a axis of the complex) and φ (the azimuthal angle of the O2 out of the plane defined by the a and b axes of the complex). This model has been successfully applied to analyze the high-resolution spectrum of O2-N2O in the region of the N2O monomer ν3 vibrational band, which will be presented in a separate paper. © 1997 American Institute of Physics.

In this paper we introduce a three-dimensional version of the Mercedes-Benz model to describe water molecules. In this model van der Waals interactions and hydrogen bonds are given explicitly through a Lennard-Jones potential and a Gaussian orientation-dependent terms, respectively. At low temperature the model freezes forming Ice-I and it reproduces the main peaks of the experimental radial distribution function of water. In addition to these structural properties, the model also captures the thermodynamical anomalies of water: The anomalous density profile, the negative thermal expansivity, the large heat capacity, and the minimum in the isothermal compressibility.

Molecular dynamics simulation has been performed on a liquid film that is sheared in between solid surfaces. As a shear is given to the liquid film, a Couette-like flow is generated in the liquid and energy conversion occurs from the macroscopic flow to the thermal energy, which is discharged back to the solid walls. In such a way, momentum and thermal energy fluxes are present simultaneously. And all these thermal and fluid phenomena take place in highly nonequilibrium state where thermal energy is not distributed equally to each degree of freedom of molecular motion in the vicinities of the solid-liquid interface. In the present paper, platinum and water are employed as solid and liquid, respectively. First, the structure and orientation of water molecules in the vicinities of the solid surfaces are analyzed and how these structure and orientation are influenced by the shear is considered. Based on this result, momentum and thermal energy transfer in the vicinities of and at the solid-liquid interfaces are investigated in detail. Results are compared with those of our previous study, in which monatomic and diatomic molecules are employed as liquid.

Dissociations after the A˜ 1B1→B˜ 1A1 photoexcitation of SiH2, SiHD, and SiD 2 were studied to investigate excited-state dynamics and effects of the initial vibrational state. The cross section (σ) for the photodissociation relative to SiH2(B˜)→Si( 1D)+H2 and the rovibrational population of the H2 fragment were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces (PESs) of the A˜ and B˜ electronic states and the transition dipole surfaces, which were reported in our previous paper [J. Chem. Phys. 122, 144307 (2005)]. The photodissociation spectrum consists of a broadband and a number of sharp peaks. For SiH2 and SiD2, the sharp peaks correspond to the resonance structure of the vibrational levels of the B˜ state and the broadbands are nearly independent of the photon energy. The broadband for SiHD increases steeply with the photon energy above 30 000 cm-1. The flux leaving the computational grid for SiH2 and SiD2 consists of at least two components, whereas that for SiHD consists of only a faster component. These large isotope effects were discussed based on the valley to the dissociation channel on PES and the difference in the position of the initial wave packet for three isotopomers.

Dissociations after the à 1B1→B 1A1 photoexcitation of SiH2, SiHD, and SiD2 were studied to investigate excited-state dynamics and effects of the initial vibrational state. The cross section (σ) for the photodissociation relative to SiH2(B)→Si(1D)+H2 and the rovibrational population of the H2 fragment were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces (PESs) of the à and B electronic states and the transition dipole surfaces, which were reported in our previous paper [J. Chem. Phys. 122, 144307 (2005)]. The photodissociation spectrum consists of a broadband and a number of sharp peaks. For SiH2 and SiD2, the sharp peaks correspond to the resonance structure of the vibrational levels of the B state and the broadbands are nearly independent of the photon energy. The broadband for SiHD increases steeply with the photon energy above 30 000 cm–1. The flux leaving the computational grid for SiH2 and SiD2 consists of at least two components, whereas that for SiHD consists of only a faster component. These large isotope effects were discussed based on the valley to the dissociation channel on PES and the difference in the position of the initial wave packet for three isotopomers.