Showing papers in "Journal of Chemical Physics in 1997"

A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics

Abstract: We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics.

Dissipative particle dynamics : bridging the gap between atomistic and mesoscopic simulation

Abstract: We critically review dissipative particle dynamics (DPD) as a mesoscopic simulation method. We have established useful parameter ranges for simulations, and have made a link between these parameters and χ-parameters in Flory-Huggins-type models. This is possible because the equation of state of the DPD fluid is essentially quadratic in density. This link opens the way to do large scale simulations, effectively describing millions of atoms, by firstly performing simulations of molecular fragments retaining all atomistic details to derive χ-parameters, then secondly using these results as input to a DPD simulation to study the formation of micelles, networks, mesophases and so forth. As an example application, we have calculated the interfacial tension σ between homopolymer melts as a function of χ and N and have found a universal scaling collapse when σ/ρkBTχ0.4 is plotted against χN for N>1. We also discuss the use of DPD to simulate the dynamics of mesoscopic systems, and indicate a possible problem with...

Continuum solvation models: A new approach to the problem of solute’s charge distribution and cavity boundaries

Abstract: In continuum solvation models the definition of a cavity that embeds the solute molecule leads to problems related to the portion of solute’s electronic charge lying outside its boundaries (charge tails). The correction strategies developed so far can be shown to work insufficiently, since they only correct the global charge defect, but lead to considerable local errors. The present paper will be focused on the theoretical and technical aspects of this problem, and it will present in detail a new method which allows a very refined treatment of solute’s charge tails in the outer space; some numerical results of solutes in water will be shown and discussed. As further analyses, the introduction of Pauli repulsion term will be considered, and the implications all these effects have on molecular properties, such as (hyper)polarizabilities, numerically evaluated. The new approach has been implemented within the framework of the polarizable continuum model (PCM).

A new definition of cavities for the computation of solvation free energies by the polarizable continuum model

Abstract: A set of rules for determining the atomic radii of spheres used to build the molecular cavities in continuum solvation models are presented. The procedure is applied to compute the hydration free energy for molecules containing H, C, N, O, F, P, S, Cl, Br, and I at a computational level (Hartree–Fock with a medium size basis set) allowing the study of relatively large systems. The optimized radii reduce the mean error with respect to the experimental solvation energies below 0.20 kcal/mol for a set of 43 neutral solutes and around 1 kcal/mol for 27 ions. Moreover the correct trends are observed for the solvation energies of homolog series, like the series ammonia–trimethylamine, that are not correctly reproduced by usual solvation models.

Basis-set convergence of correlated calculations on water

Abstract: The basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mo/ller–Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mEh by means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances rij. For the extrapolations to the basis-set limit of the correlation energies, fits of the form a+bX−3 (where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported.

Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation

Abstract: A set of 148 molecules having well-established enthalpies of formation at 298 K is presented. This set, referred to as the G2 neutral test set, includes the 55 molecules whose atomization energies were used to test Gaussian-2 ~G2! theory @J. Chem. Phys. 94, 7221 ~1991!# and 93 new molecules. The G2 test set includes 29 radicals, 35 nonhydrogen systems, 22 hydrocarbons, 47 substituted hydrocarbons, and 15 inorganic hydrides. It is hoped that this new test set will provide a means for assessing and improving new theoretical models. From an assessment of G2 and density functional theories ~DFT! on this test set it is found that G2 theory is the most reliable method both in terms of average absolute deviation ~1.58 kcal/mol! and maximum deviation ~8.2 kcal/mol!. The largest deviations between experiment and G2 theory occur for molecules having multiple halogens. Inclusion of spin‐orbit effects reduces the average absolute deviation to 1.47 kcal/mol and significantly improves the results for the chlorine substituted molecules, but little overall improvement is seen for the fluorine substituted molecules. Of the two modified versions of G2 theory examined in this study, G2~MP2,SVP! theory ~average absolute deviation51.93 kcal/mol! performs better than G2~MP2! theory ~2.04 kcal/mol!. The G2~MP2,SVP! theory is found to perform very well for hydrocarbons, radicals, and inorganic hydrides. Of the seven DFT methods investigated, the B3LYP method has the smallest average absolute deviation ~3.11 kcal/mol!. It also has a significantly larger distribution of error than the G2 methods with a maximum deviation of 20.1 kcal/mol. © 1997 American Institute of Physics.@S0021-9606~97!02202-2#

Density-functional thermochemistry. V. Systematic optimization of exchange-correlation functionals

Abstract: A systematic procedure for refining gradient corrections in Kohn–Sham exchange-correlation functionals is presented. The procedure is based on least-squares fitting to accurate thermochemical data. In this first application of the method, we use the G2 test set of Pople and co-workers to generate what we believe to be an optimum GGA/exact-exchange density-functional theory (i.e., generalized gradient approximation with mixing of exactly computed exchange).

A simple nonequilibrium molecular dynamics method for calculating the thermal conductivity

Abstract: A nonequilibrium molecular dynamics method for calculating the thermal conductivity is presented. It reverses the usual cause and effect picture. The “effect,” the heat flux, is imposed on the system and the “cause,” the temperature gradient is obtained from the simulation. Besides being very simple to implement, the scheme offers several advantages such as compatibility with periodic boundary conditions, conservation of total energy and total linear momentum, and the sampling of a rapidly converging quantity (temperature gradient) rather than a slowly converging one (heat flux). The scheme is tested on the Lennard-Jones fluid.

The determination of an accurate isotope dependent potential energy surface for water from extensive ab initio calculations and experimental data

Abstract: We report on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base with J⩽5 for H216O The changes in the PES are small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with the experiment Using this adjusted PES, we can match 30 092 of the 30 117 transitions in the HITRAN 96 data base for H216O with theoretical lines The 10, 25, 50, 75, and 90 percentiles of the difference between the calculated and tabulated line positions are −011, −004, −001, 002, and 007 cm−1 Nonadiabatic effects are not explicitly included About 3% of the tabulated line positions appear to be incorrect Similar agreement using this adjusted PES is obtained for the 17O and 18O isotopes For HD16O, the agreement is not as good, with a root-mean-square error of 025 cm−1 for line

Statistical associating fluid theory for chain molecules with attractive potentials of variable range

Abstract: A version of the statistical associating fluid theory (SAFT) is developed for chain molecules of hard-core segments with attractive potentials of variable range (SAFT-VR). The different contributions to the Helmholtz free energy are evaluated according to the Wertheim perturbation theory. The monomer properties are obtained from a high-temperature expansion up to second order, using a compact expression for the first-order perturbation term (mean-attractive energy) a1. Making use of the mean-value theorem, a1 is given as the van der Waals attractive constant and the Carnahan and Starling contact value for the hard-sphere radial distribution function in terms of an effective packing fraction. The second-order perturbation term a2 is evaluated with the local compressibility approximation. The monomer cavity function, required for the calculation of the free energy due to the formation of the chains and the contribution due to association, is given as a function of a1. We analyse the equation of state for ch...

The topology of multidimensional potential energy surfaces: Theory and application to peptide structure and kinetics

Abstract: Topological characteristics of multidimensional potential energy surfaces are explored and the full conformation space is mapped on the set of local minima. This map partitions conformation space into energy-dependent or temperature-dependent “attraction basins’’ and generates a “disconnectivity’’ graph that reflects the basin connectivity and characterizes the shape of the multidimensional surface. The partitioning of the conformation space is used to express the temporal behavior of the system in terms of basin-to-basin kinetics instead of the usual state-to-state transitions. For this purpose the transition matrix of the system is expressed in terms of basin-to-basin transitions and the corresponding master equation is solved. As an example, the approach is applied to the tetrapeptide, isobutyryl-(ala)3-NH-methyl (IAN), which is the shortest peptide that can form a full helical turn. A nearly complete list of minima and barriers is available for this system from the work of Czerminiski and Elber. The multidimensional potential energy surface of the peptide is shown to exhibit an overall “funnel’’ shape. The relation between connectivity and spatial proximity in dihedral angle space is examined. It is found that, although the two are similar, closeness in one does not always imply closeness in the other. The basin to basin kinetics is examined using a master equation and the results are interpreted in terms of kinetic connectivity. The conformation space of the peptide is divided up in terms of the surface topography to model its “folding’’ behavior. Even in this very simple system, the kinetics exhibit a “trapping’’ state which appears as a “kinetic intermediate,’’ as in the folding of proteins. The approach described here can be used more generally to classify multidimensional potential energy surfaces and the time development of complex systems.

Tracing the phase boundaries of hard spherocylinders

Abstract: We have mapped out the complete phase diagram of hard spherocylinders as a function of the shape anisotropy L/D. Special computational techniques were required to locate phase transitions in the limit L/D→∞ and in the close-packing limit for L/D→0. The phase boundaries of five different phases were established: the isotropic fluid, the liquid crystalline smectic A and nematic phases, the orientationally ordered solids—in AAA and ABC stacking—and the plastic or rotator solid. The rotator phase is unstable for L/D⩾0.35 and the AAA crystal becomes unstable for lengths smaller than L/D≈7. The triple points isotropic-smectic-A-solid and isotropic-nematic-smectic-A are estimated to occur at L/D=3.1 and L/D=3.7, respectively. For the low L/D region, a modified version of the Gibbs–Duhem integration method was used to calculate the isotropic-solid coexistence curves. This method was also applied to the I-N transition for L/D>10. For large L/D the simulation results approach the predictions of the Onsager theory. ...

The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state

Abstract: We study the luminescence of surface modified CdSe nanocrystallites There has been much speculation as to the origin of the band edge emission in these quantum confined structures Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments Our results are compared with theoretical predictions for the nonresonant and resonant luminescence We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-eth

Molecular dynamics study of water clusters, liquid, and liquid-vapor interface of water with many-body potentials

Abstract: The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structuralproperties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of vaporization, as well as the diffusion coefficient, are in excellent agreement with experimental values. Structuralproperties of liquidwater, such as the radial distribution functions, neutron, and x-ray scattering intensities, were calculated and critically evaluated against the experimental measurements. In all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profile of the water’s liquid/vapor interface shows that the interface is not sharp at a microscopic level and has a thickness of 3.2 A at 298 K. These results are consistent with those reported in earlier work on the same systems. The calculated surface tension at room temperature is in reasonable agreement with the corresponding experimental data. As expected, the computed average dipole moments of water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values.

A method for accelerating the molecular dynamics simulation of infrequent events

Abstract: For infrequent-event systems, transition state theory (TST) is a powerful approach for overcoming the time scale limitations of the molecular dynamics (MD) simulation method, provided one knows the locations of the potential-energy basins (states) and the TST dividing surfaces (or the saddle points) between them. Often, however, the states to which the system will evolve are not known in advance. We present a new, TST-based method for extending the MD time scale that does not require advanced knowledge of the states of the system or the transition states that separate them. The potential is augmented by a bias potential, designed to raise the energy in regions other than at the dividing surfaces. State to state evolution on the biased potential occurs in the proper sequence, but at an accelerated rate with a nonlinear time scale. Time is no longer an independent variable, but becomes a statistically estimated property that converges to the exact result at long times. The long-time dynamical behavior is ex...

Investigation of the temperature dependence of dielectric relaxation in liquid water by thz reflection spectroscopy and molecular dynamics simulation

Abstract: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.

Site–site pair correlation functions of water from 25 to 400 °C: Revised analysis of new and old diffraction data

Abstract: Recent controversy in the literature about the structure of water away from ambient conditions is manifested by significant differences between the site–site pair correlation functions of water as derived from neutron diffraction data and the same quantities obtained in computer simulations using an effective pairwise potential for the intermolecular interactions. One possible explanation of the discrepancies between computer models of water and neutron diffraction results near the critical point is that they arise from uncertainties in the inelasticity correction, which is particularly large for light water. To test out this idea, a new method of obtaining the pair correlations is described. This new analysis method is applied both to the earlier neutron data on non-ambient water and on recently reported data for a range of densities at 573 K. For this newer data the incident neutron spectrum is derived from an ambient water moderator instead of the liquid methane (100 K) moderator of previous work, and ...

Liquid-Liquid Phase Separation in Supersaturated Lysozyme Solutions and Associated Precipitate Formation/Crystallization

Abstract: Using cloud point determinations, the phase boundaries (binodals) for metastable liquid-liquid (L-L) separation in supersaturated hen egg white lysozyme solutions with 3%, 5%, and 7% (wlv) NaCl at pH= 4.5 and protein concentrations c between 40 and 400 mg/ml were determined. The critical temperature for the binodal increased approximately linearly with salt concentration. The coexisting liquid phases both remained supersaturated but differed widely in protein concentration. No salt repartitioning was observed between the initial and the two separated liquid phases. After the L-L separation, due to the presence of the high protein concentration phase, crystallization occurred much more rapidly than in the initial solution. At high initial protein concentrations, a metastable gel phase formed at temperatures above the liquid binodal. Both crystal nucleation and gel formation were accelerated in samples that had been cycled through the binodal. Solutions in the gel and L-L regions yielded various types of precipitates. Based on theoretical considerations, previous observations with other proteins, and our experimental results with lysozyme, a generic phase diagram for globular proteins is put forth. A limited region in the (T,c) plane favorable for the growth of protein single crystals is delineated.

Harmonic inversion of time signals and its applications

Abstract: New methods of high resolution spectral analysis of short time signals are presented. These methods utilize the filter-diagonalization approach of Wall and Neuhauser [J. Chem. Phys. 102, 8011 (1995)] that extracts the complex frequencies ωk and amplitudes dk from a signal C(t)=∑kdke−itωk in a small frequency interval by recasting the harmonic inversion problem as the one of a small matrix diagonalization. The present methods are rigorously adapted to the conventional case of the signal available on a sparse equidistant time grid and use a more efficient boxlike filter. Various applications are discussed, such as iterative diagonalization of large Hamiltonian matrices for calculating bound and resonance states, scattering calculations in the presence of narrow resonances, etc. For the scattering problem the harmonic inversion is directly applied to the signal cn=(χf,Tn(Ĥ)χi), generated by the dynamical system governed by a modified Chebyshev recursion, avoiding the usual recasting the problem to the time d...

Vibrational self-consistent field method for many-mode systems: A new approach and application to the vibrations of CO adsorbed on Cu(100)

Abstract: We report calculations of the vibrational energies of CO–Cu(100) using a new code to perform vibrational self-consistent field (VSCF) and state-mixing calculations for many-mode systems. The major new feature of the code is the representation of the potential. Unlike recent implementations of the VSCF method, the potential is not expanded in terms of normal coordinates as a multinomial series about a minimum. The full potential, in normal coordinates, is used in the Watson Hamiltonian. This approach, while rigorous, can lead to prohibitively large numerical quadratures, and so we suggest a novel representation of the potential as an expansion in all two-mode, or all three-mode, or all four-mode coupling terms. The new code is tested against previous exact calculations of vibrational states of HCO, and also against previous VSCF calculations that used a fourth-order, normal coordinate force field representation of the global HCO potential. The new code is applied to calculations of the vibrations of CO ads...

Calculation of electronic coupling matrix elements for ground and excited state electron transfer reactions: Comparison of the generalized Mulliken-Hush and block diagonalization methods

Abstract: Two independent methods are presented for the nonperturbative calculation of the electronic coupling matrix element (Hab) for electron transfer reactions using ab initio electronic structure theory. The first is based on the generalized Mulliken–Hush (GMH) model, a multistate generalization of the Mulliken Hush formalism for the electronic coupling. The second is based on the block diagonalization (BD) approach of Cederbaum, Domcke, and co-workers. Detailed quantitative comparisons of the two methods are carried out based on results for (a) several states of the system Zn2OH2+ and (b) the low-lying states of the benzene–Cl atom complex and its contact ion pair. Generally good agreement between the two methods is obtained over a range of geometries. Either method can be applied at an arbitrary nuclear geometry and, as a result, may be used to test the validity of the Condon approximation. Examples of nonmonotonic behavior of the electronic coupling as a function of nuclear coordinates are observed for Zn2OH2+. Both methods also yield a natural definition of the effective distance (rDA) between donor (D) and acceptor (A) sites, in contrast to earlier approaches which required independent estimates of rDA, generally based on molecular structure data.

The CC3 model: An iterative coupled cluster approach including connected triples

Abstract: An alternative derivation of many-body perturbation theory (MBPT) has been given, where a coupled cluster parametrization is used for the wave function and the method of undetermined Lagrange multipliers is applied to set up a variational coupled cluster energy expression. In this variational formulation, the nth-order amplitudes determine the energy to order 2n+1 and the nth-order multipliers determine the energy to order 2n+2. We have developed an iterative approximate coupled cluster singles, doubles, and triples model CC3, where the triples amplitudes are correct through second order and the singles amplitudes are treated without approximations due to the unique role of singles as approximate orbital relaxation parameters. The compact energy expressions obtained from the variational formulation exhibit in a simple way the relationship between CC3, CCSDT-1a [Lee et al., J. Chem. Phys. 81, 5906 (1984)] CCSDT-1b models [Urban et al., J. Chem. Phys. 83, 4041 (1985)], and the CCSD(T) model [Raghavachari et al., Chem. Phys. Lett. 157, 479 (1989)]. Sample calculations of total energies are presented for the molecules H2O, C2, CO, and C2H4. Comparisons are made with full CCSDT, CCSDT-1a, CCSDT-1b, CCSD(T), and full configuration interaction (FCI) results. These calculations demonstrate that CC3 and CCSD(T) give total energies of a similar quality. If results obtained by CC3 and CCSD(T) differ significantly, neither method can be trusted. In contrast to CCSD(T), time-dependent response functions can be obtained for CC3.

Block copolymer microstructures in the intermediate-segregation regime

Abstract: A detailed examination of the intermediate-segregation regime of diblock copolymer melts is presented using the incompressible Gaussian chain model and self-consistent field theory (SCFT). We find that the competition between interfacial tension and chain stretching used to describe behavior in the strong-segregation regime also explains behavior in this regime. Phase transitions from lamellae (L) to cylinders (C) to spheres (S) occur due to the spontaneous curvature produced as the asymmetry in the diblock composition increases. Complex phases, gyroid (G), perforated lamellar (PL), and double diamond (D), have curvatures between those of L and C, and therefore they compete for stability along the L/C boundary. Nevertheless, only G exhibits a region of stability. To explain why, we recognize that interfacial tension prefers the formation of constant mean curvature (CMC) surfaces to reduce interfacial area, and chain stretching favors domains of uniform thickness so as to avoid packing frustration. While t...

Statistical modeling of collision-induced dissociation thresholds

Abstract: Analysis of the energy dependence of the cross sections for collision-induced dissociation reactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energy-dependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li+(ROH...

Thin films of block copolymer

Abstract: We develop a numerical method for examining complex morphologies in thin films of block copolymer using self-consistent field theory Applying the method to confined films of symmetric diblock copolymer, we evaluate the stability of parallel, perpendicular, and mixed lamellar phases In general, lamellar domains formed by the diblocks are oriented parallel to the film by surface fields However, their orientation can flip to perpendicular when the natural period of the lamellae is incommensurate with the film thickness Experiments and Monte Carlo simulations have indicated that mixed lamellar phases may also occur, but for symmetric diblocks, we find these phases to be slightly unstable relative to perpendicular lamellae Nevertheless, just a small asymmetry in the molecule stabilizes a mixed lamellar phase Although our work focuses on confined films, we do discuss the behavior that results when films are unconfined

Dynamics of glass-forming liquids. III. Comparing the dielectric α- and β-relaxation of 1-propanol and o-terphenyl

Abstract: We have measured the dielectric relaxation of the glass-former 1-propanol for temperatures between 65 and 350 K in the frequency range 10−2 to 2⋅1010 Hz and the photon correlation spectro-scopy decays near Tg. Attributing the strong Debye-type process of 1-propanol to distinct -OH group effects leaves two faster processes related to the structural relaxation which can be identified as α-relaxation and Johari–Goldstein type β-relaxation characteristic of nonhydrogen-bonding supercooled liquids. From the temperature dependent relaxation times τ(T) regarding the three distinct loss peaks, we can specify an α-β-bifurcation temperature Tβ, which coincides with characteristic qualitative changes in the τ(T) behavior, as also observed for ortho-terphenyl and other glass-forming liquids. This assignment is confirmed by the correla-tion times derived from incoherent quasielastic light-scattering data obtained from the simultaneously measured photon-correlation spectroscopy.

Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy

Abstract: The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps.

The dynamic mean-field density functional method and its application to the mesoscopic dynamics of quenched block copolymer melts

Abstract: 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.

From clusters to bulk: A relativistic density functional investigation on a series of gold clusters Aun, n=6,…,147

Abstract: A series of gold clusters spanning the size range from Au6 through Au147 (with diameters from 0.7 to 1.7 nm) in icosahedral, octahedral, and cuboctahedral structure has been theoretically investigated by means of a scalar relativistic all-electron density functional method. One of the main objectives of this work was to analyze the convergence of cluster properties toward the corresponding bulk metal values and to compare the results obtained for the local density approximation (LDA) to those for a generalized gradient approximation (GGA) to the exchange-correlation functional. The average gold–gold distance in the clusters increases with their nuclearity and correlates essentially linearly with the average coordination number in the clusters. An extrapolation to the bulk coordination of 12 yields a gold–gold distance of 289 pm in LDA, very close to the experimental bulk value of 288 pm, while the extrapolated GGA gold–gold distance is 297 pm. The cluster cohesive energy varies linearly with the inverse o...

Interfacing relativistic and nonrelativistic methods. i. normalized elimination of the small component in the modified dirac equation

Abstract: The introduction of relativistic terms into the nonrelativistic all-electron Schrodinger equation is achieved by the method of normalized elimination of the small component (ESC) within the matrix representation of the modified Dirac equation. In contrast to the usual method of ESC, the method presented retains the correct relativistic normalization, and permits the construction of a single matrix relating the large and small component coefficient matrices for an entire set of positive energy one-particle states, thus enabling the whole set to be obtained with a single diagonalization. This matrix is used to define a modified set of one- and two-electron integrals which have the same appearance as the nonrelativistic integrals, and to which they reduce in the limit α→0. The normalized method corresponds to a projection of the Dirac–Fock matrix onto the positive energy states. Inclusion of the normalization reduces the discrepancy between the eigenvalues of the ESC approach and the Dirac eigenvalues for a ...