Volume 126, Issue 10, 14 March 2007
 COMMUNICATIONS


Microwave observation of the “recently found” polar OCS dimer
View Description Hide DescriptionRecently Afshari et al. reported on the detection of a new infrared band which was assigned to the “longanticipated polar isomer of the OCS dimer” [J. Chem. Phys.126, 071102 (2007)]. The authors report here the microwave confirmation of their results. The lowest energy, nonpolar isomer of has long been known from IR spectroscopy, while the polar form has only been deduced from qualitative beam refocusing experiments. The higher energy, polar isomer of has been produced by high pressure expansion of dilute OCS in helium. A surprisingly strong microwave spectrum of has been observed and assigned.

Picosecond water dynamics adjacent to charged paramagnetic ions measured by magnetic relaxation dispersion
View Description Hide DescriptionMeasurements of waterproton spinlattice relaxation rate constants as a function of magnetic field strength [magnetic relaxationdispersion (MRD)] in aqueous solutions of paramagneticsolutes reveal a peak in the MRD profile. These previously unobserved peaks require that the time correlation functions describing the waterprotonelectron dipolar coupling have a periodic contribution. In aqueous solutions of iron(III) ion the peak corresponds to a frequency of , which the authors ascribe to the motion of water participating in the second coordination sphere of the triply charged solute ion. Similar peaks of weaker intensity in the same time range are observed for aqueous solutions of chromium(III) chloride as well as for ion pairs formed by ammonium ion with trioxalatochromate(III) ion. The widths of the dispersion peaks are consistent with a lifetime for the periodic motion in the range of or longer.
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 ARTICLES

 Theoretical Methods and Algorithms

Langevin stabilization of moleculardynamics simulations of polymers by means of quasisymplectic algorithms
View Description Hide DescriptionAlgorithms for the numerical integration of Langevin equations are compared in detail from the point of view of their accuracy, numerical efficiency, and stability to assess them as potential candidates for moleculardynamics simulations of polymeric systems. Some algorithms are symplectic in the deterministic frictionless limit and prove to stabilize long timestep integrators. They are tested against other popular algorithms. The optimal algorithm depends on the main goal: accuracy or efficiency. The former depends on the observable of interest. A recently developed quasisymplectic algorithm with great accuracy in the position evaluation exhibits better overall accuracy and stability than the other ones. On the other hand, the wellknown BrünGerBrooksKarplus [Chem. Phys. Lett.105, 495 (1982)] algorithm is found to be faster with limited accuracy loss but less stable. It is also found that using higherorder algorithms does not necessarily improve the accuracy. Moreover, they usually require more force evaluations per single step, thus leading to poorer performances.

Density functionals that are one and two are not always manyelectron selfinteractionfree, as shown for , , , and
View Description Hide DescriptionThe common density functionals for the exchangecorrelation energy make serious selfinteraction errors in the molecular dissociation limit when real or spurious noninteger electron numbers are found on the dissociation products. An “electron selfinteractionfree” functional for positive integer is one that produces a realistic linear variation of total energy with in the range of , and so can avoid these errors. This desideratum is a natural generalization to all of the more familiar one of oneelectron selfinteraction freedom. The intent of this paper is not to advocate for any functional, but to understand what is required for a functional to be electron selfinteractionfree and thus correct even for highly stretched bonds. The original PerdewZunger selfinteraction correction (SIC) and our scaleddown variant of it are exactly one and nearly twoelectron selfinteractionfree, but only the former is nearly so for atoms with . Thus all these SIC’s produce an exact binding energy curve for , and an accurate one for , but only the unscaled PerdewZunger SIC produces an accurate one for , where there are more than two electrons on each fragment . We also discuss , which is relatively free from selfinteraction errors. We suggest that the ability of the original and unscaled PerdewZunger SIC to be nearly electron selfinteractionfree for atoms of all stems in part from its formal resemblance to the HartreeFock theory, with which it shares a sum rule on the exchangecorrelation hole of an open system.

Resonance of relaxation time in the temperature modulated Schlögl model
View Description Hide DescriptionWe show the possibility to accelerate—in a resonant way—a nonlinear chemical reaction by imposing a small temperature modulation. This classical resonance, which happens for particular modulation frequencies, is illustrated on the athermic cubic Schlögl model, which allows us to get analytical expressions for both the reactionrelaxation time and the frequencyresonant delay.

SASS: A symmetry adapted stochastic search algorithm exploiting site symmetry
View Description Hide DescriptionA simple symmetry adapted search algorithm (SASS) exploiting point group symmetry increases the efficiency of systematic explorations of complex quantum mechanical potential energy surfaces. In contrast to previously described stochastic approaches, which do not employ symmetry, candidate structures are generated within simple point groups, such as , , and . This facilitates efficient sampling of the Pople's dimensional configuration space and increases the speed and effectiveness of quantum chemical geometryoptimizations. Pople's concept of framework groups [J. Am. Chem. Soc.102, 4615 (1980)] is used to partition the configuration space into structures spanning all possible distributions of sets of symmetry equivalent atoms. This provides an efficient means of computing all structures of a given symmetry with minimum redundancy. This approach also is advantageous for generating initial structures for global optimizations via genetic algorithm and other stochastic global search techniques. Application of the SASS method is illustrated by locating 14 lowlying stationary points on the ccpwCVDZ ROCCSD(T) potential energy surface of . The global minimum structure is identified, along with many unique, nonintuitive, energetically favorable isomers.

Interpolating moving leastsquares methods for fitting potential energy surfaces: An application to the unimolecular reaction
View Description Hide DescriptionClassical trajectories have been used to compute rates for the unimolecular reaction on a fitted ab initio potential energy surface (PES). The ab initio energies were obtained from CCSD(T)/augccpvtz electronic structure calculations. The ab initio energies were fitted by the interpolating moving leastsquares (IMLS) method. This work continues the development of the IMLS method for producing ab initio PESs for use in molecular dynamics simulations of manyatom systems. A duallevel scheme was used in which the preliminary selection of data points was done using a lowlevel theory and the points used for fitting the final PES were obtained at the desired higher level of theory. Classical trajectories were used on various lowlevel IMLS fits to tune the fit to the unimolecular reaction under study. Procedures for efficiently picking data points, selecting basis functions, and defining cutoff limits to exclude distant points were investigated. The accuracy of the fitted PES was assessed by comparing interpolated values of quantities to the corresponding ab initio values. With as little as 330 ab initio points classical trajectory rate constants were converged to 5%–10% and the rms error over the sixdimensional region sampled by the trajectories was a few tenths of a kcal/mol.

Improved diffusion Monte Carlo for bosonic systems using timestep extrapolation “on the fly”
View Description Hide DescriptionA diffusionMonte Carlo algorithm employing “on the fly” extrapolation with respect to the time step is implemented and demonstrated simulating realistic systems. Significant advantages are obtained when using on the fly extrapolation, leading to reduced systematic and statistical errors. The sound theoretical basis of extrapolation on the fly is discussed and compared to justifications for the a posteriori extrapolation.

Efficient vector potential method for calculating electronic and nuclear response of infinite periodic systems to finite electric fields
View Description Hide DescriptionThe response of periodic systems to external electric fields is a challenging theoretical problem. The authors show how the vector potential approach yields a numerically efficient treatment of the combined electronic and nuclear response to a finite static field. Their method is based on a selfconsistent reformulation of the charge flow term in the single particle Hamiltonian. Careful numerical implementation yields a treatment whose computational needs are only marginally larger than those of a conventional fieldfree calculation. To prove the method exemplary polymer calculations are done for a model Hamiltonian. The latter contains all essential elements of an ab initio KohnSham or HartreeFock Hamiltonian but allows for extensive testing. The extension to threedimensional systems is described.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Electric deflection studies of rhodium clusters
View Description Hide DescriptionThe static electric dipole polarizabilities of rhodium clusters , , have been measured via a molecular beam deflection method. Uniform highfield beam deflections, indicative of induced polarization, were observed for all except and which by contrast exhibited beam broadening and anomalously high effective polarizabilities. Analysis of the beam deflection profile of indicates that it possesses a permanent dipole moment of . Unlike the other clusters in the size range, the polarizability of is observed to decrease with increasing source temperature. We attribute this temperature dependence to paraelectric behavior, suggesting that is a fluxional molecule possessing a dipole moment that spatially fluctuates, uncorrelated with overall rotation.

Field shifts and lowest order QED corrections for the ground and states of the helium atoms
View Description Hide DescriptionThe bound state properties of the ground state and the lowest triplet state of the , , and helium atoms are determined to very high accuracy from the results of direct numerical computations. To compute the bound state properties of these atoms the author applied his exponential variational expansion in relative/perimetric threebody coordinates. For the ground state and the lowest triplet state of the , , and atoms the author also determined the lowest order QED corrections and the field component of isotopic shift ( shift). For the state of the atom the hyperfine structure splitting is evaluated. The considered properties of the ground state and the lowest state in the and atoms are of great interest in a number of applications.

Single molecule photon emission statistics of driven threelevel systems
View Description Hide DescriptionWe study the statistics of photon emission from threelevel single molecule systems. The generating function method [Y. Zheng and F. L. H. Brown, Phys. Rev. Lett.90, 238305 (2003)] is used to calculate steady state absorption line shapes and Mandel’s parameter as a function of excitation frequency, as well as the time dependence associated with approach to the steady state. The line shape calculations confirm known results derived via other methods, while the parameter results display complex frequency dependences not amenable to simple interpretation. This study confirms the applicability of the generating function formalism to multilevel quantum systems, including the proper modeling of quantum coherence effects.

Enhancement of anthracene fragmentation by circularly polarized intense femtosecond laser pulse
View Description Hide DescriptionThe authors compared circularly and linearly polarized lights in the ionization and fragmentation of anthracene, using femtosecond laser pulses at intensities of . Singly and doubly charged intact molecular ions as well as numerous fragment ions were observed in the mass spectra, which were investigated as a function of laser intensity and polarization. At comparable intensities above the saturation threshold for complete ionization, the fragmentation pathways are enhanced with a circularly polarized field compared to a linearly polarized field. Resonant excitation of the molecular cation through the transition is proposed to be the initial step to ion fragmentation. The circularly polarized field interacts with a larger fraction of the randomly oriented molecules than the linearly polarized field, and this is considered to be the reason for the enhanced fragmentation brought about by circularly polarized light.

Role of the electric dipole moment in positron binding to the ground and excited states of the BeO molecule
View Description Hide DescriptionSelfconsistentfield and multireference single and doubleexcitation configuration interaction (CI) calculations have been carried out for various electronic states of the beryllium oxide molecule and their positronattached counterparts. Particular emphasis is placed on the correlation between the polarity of a given BeO state and the magnitude of the positron binding energy as the internuclear distance is varied. Potential curves are computed for all BeO states that correlate with the first three atomic limits for this system and good agreement is found between the experimental and calculated spectroscopic constants in all cases. The present level of CI treatment is known to underestimate the positron affinities of atoms by at least several tenths of an eV, and this fact needs to be taken into account in evaluating the results for positron binding to molecules. The lowest BeO excited states are not found to bind with a positron in the FranckCondon region due to their comparatively small dipole moments caused by O to Be charge transfer relative to the ground state, which in turn does have a fairly sizeable positron affinity. The situation changes significantly as dissociation proceeds, however, with both and positronic states lying several tenths of an eV lower than their neutral counterparts over a broad range of internuclear distance.

Dynamics of the Rydberg electron in reactive collisions
View Description Hide DescriptionExperimental crossedbeam studies carried out previously have indicated that the dynamics of the Rydbergatommolecule reaction are very similar to those of the corresponding ionmolecule reaction. The equivalence of the cross sections for these related systems would open up a new approach to the experimental study of ionmolecule reactions. However, a recent experimental and theoretical study has brought to light some important qualitative differences between the Rydbergatom reaction and the ionmolecule reaction; in particular, the experimental cross section for the Rydbergatom reaction exhibits a higher degree of forwardbackward scattering asymmetry than predicted by a quasiclassical trajectory study of the ionmolecule reaction. In this paper, the authors consider the dynamics of the Rydbergelectron over the course of a reactive collision and the implications of these dynamics for the Rydbergatommolecule crossedbeam experiment. Using an approach based on perturbation theory, they estimate the attenuation of the experimental signal due to the Rydbergelectron dynamics as a function of the scattering angle. They show that at least part of the experimental asymmetry can be ascribed to this angle dependent attenuation. Their results offer general insight into the practical aspects of the experimental study of ionmolecule reactions by means of their Rydbergatom counterparts.

Theoretical investigation of the alkalineearth dihydrides from relativistic allelectron, pseudopotential, and densityfunctional study
View Description Hide DescriptionHighly precise ground state geometries, harmonic vibrational frequencies and force constants of alkalineearth dihydrides from to are obtained using relativistic smallcore energyconsistent effective core potentials at the coupledcluster level. The results are compared with allelectron as well as density functional calculations. Allelectron results, in particular, clearly show the importance of relativistic effects in the properties considered in this paper. The monotonic trends in the geometries are explained in terms of secondorder perturbation theory. Trends in the force constants are monotonic except for the bending mode where an anomaly occurs from to . It is rationalized in terms of reduced hybridization due to relativity, which is shown to be an energy effect attributed to the stabilization of the orbital. The pseudopotentials show an excellent performance in comparison with allelectron methods and are therefore successfully transferred to molecular cases. The density functional methods, however, suffer from functional dependencies with B3LYP performing the best in this case.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Probe particles alter dynamic heterogeneities in simple supercooled systems
View Description Hide DescriptionThe authors present results from molecular dynamics simulations on the effect of smooth and rough probes on the dynamics of a supercooled LennardJones (LJ) mixture. The probe diameter was systematically varied from one to seven times the diameter of the large particles of the LJ mixture. Mean square displacements show that in the presence of a large smooth probe the supercooled liquid speeds up, while in the presence of a large rough probe, the supercooled liquid slows down. NonGaussian parameters indicate that with both smooth and rough probes, the heterogeneity of the supercooled system increases. From the analysis of local DebyeWaller factors, it is evident that the change in the dynamics of the LJ system is heterogeneous, with the largest perturbations close to the probes. Large smooth and rough probes appear to set up heterogeneities in these supercooled systems that would otherwise not occur, and these heterogeneities persist for long times.

Nonideal gas solvation thermodynamics
View Description Hide DescriptionGeneral expressions are obtained for the thermodynamic properties of nonideal gases of arbitrary composition. These include the equation of state and solvation thermodynamic functions of mixtures with any number of components, expanded to first order in total number density. The results are expressed in terms of binary second virial coefficients which are in turn related to binary interaction potential energy functions. Solvation thermodynamic functions (at both constant pressure and constant volume) are decomposed into solutesolvent and solventreorganization contributions, thus indicating how each of the latter quantities may be experimentally measured. Moreover, the results are used to identify soluteinduced changes in solventthermodynamic functions, as well as nonideal (excess) contributions to chemical reaction equilibria and solvation heat capacities. The effects of a solute on the reorganization energy and chemical potential of a nonideal solvent are shown to differ qualitatively from both the corresponding ideal gas and high density fluid results.

Comparing calorimetric and dielectric polarization modes in viscous 2ethyl1hexanol
View Description Hide DescriptionDielectric relaxation and dynamic heat capacity measurements are compared for 2ethyl1hexanol near its glass transition temperature in order to further clarify the origin of the prominent Debyetype loss peak observed in many monohydroxy alcohols and other hydrogenbonding liquids. While the dielectric spectrum displays two distinct polarization processes that are separated by a factor of 2000 in terms of the peak frequency, the heat capacity shows only a single peak. The dielectric process with lower amplitude and higher peak frequency coincides with the calorimetric signal, whereas the large dielectric Debye signal is not associated with calorimetric modes. The authors conclude that the Debye process corresponds to a transition among states which differ in energy only in the case of an external electric field.

Effects of a constant electric field on the diffusional instability of cubic autocatalytic reaction fronts
View Description Hide DescriptionAn electric field applied in the direction of propagation of a chemical reactiondiffusion front can affect the stability of this front with regard to diffusive instabilities. The influence of an applied constant electric field is investigated by a linear stability analysis and by nonlinear simulations of a simple chemical system based on the cubic autocatalytic reaction. The diffusional stability of the front is seen to depend on the intensity and sign of the applied field, and , the ratio diffusion coefficients of the reactant species. Depending on , the front can become more or less diffusively unstable for a given value of . Above a critical value of , which depends on , electrophoretic separation of the two fronts is observed.