Volume 123, Issue 5, 01 August 2005
 COMMUNICATIONS


Two photoionization thresholds of produced by photodissociation at 248 nm: Further evidence for cyclic
View Description Hide DescriptionWe present results of nearthreshold photoionization of photofragments produced by laser photodissociation of at 248 nm. The time of flight of recoiling is used to resolve two photochemical channels producing , which exhibit different translational energy release. The two forms of resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear and cyclic . This result agrees with the existing theoretical calculations of excited and ionic states of and strengthens previous experimental results which suggested that the photolysis produces a cyclic form of .

The surprisingly short vibrational lifetime of the internal stretch of CO adsorbed on Si(100)
View Description Hide DescriptionPicosecond sumfrequency generation spectroscopy has been employed to study the dynamics of the internal stretch vibration of CO adsorbed on a Si(100) surface. Using the IR pump—sumfrequency generation probe method, the vibrational lifetime of the C–O stretch vibration has been determined to be . Within the experimental error limits, the identical lifetime was observed for and . No strong dependency on the carrier density in the substrate, inferred from measurements using differently doped crystals, was observed.

Magnetoresistance devices based on singlewalled carbon nanotubes
View Description Hide DescriptionWe demonstrate the physical principles for the construction of a nanometersized magnetoresistancedevice based on the AharonovBohm effect [Phys. Rev.115, 485 (1959)]. The proposed device is made of a short singlewalled carbon nanotube(SWCNT) placed on a substrate and coupled to a tip/contacts. We consider conductance due to the motion of electrons along the circumference of the tube (as opposed to the motion parallel to its axis). We find that the circumference conductance is sensitive to magnetic fields threading the SWCNT due to the AharonovBohm effect, and show that by retracting the tip/contacts, so that the coupling to the SWCNT is reduced, very high sensitivity to the threading magnetic field develops. This is due to the formation of a narrow resonance through which the tunneling current flows. Using a bias potential the resonance can be shifted to low magnetic fields, allowing the control of conductance with magnetic fields of the order of 1 T.

Endothermic freezing on heating and exothermic melting on cooling
View Description Hide DescriptionGenerally, a liquid freezes exothermally on cooling and a crystal melts endothermally on heating. Here we report an opposite occurrence—a liquid’sendothermic freezing on heating and the resulting crystal’s exothermicmelting on cooling at ambient pressures. decreases on freezing and increases on melting, and the equilibrium temperature meets the thermodynamic requirement. Melting on cooling takes longer than freezing on heating. A rapidly cooled crystal state becomes kinetically frozen, evocative of a nonergodic state. Both and enthalpy relax like those of glasses, though the viscosity is only a few centipoise. The crystal state belongs to energy minima higher than those of the melt, which has consequences for the use of potentialenergy landscape, or inherent structures, for a thermodynamic description of a material.

Threephoton nearthreshold photoionization dynamics of isooctane
View Description Hide DescriptionThe electron survival probability following threephoton ( total) nearthreshold photoionization of neat isooctane is measured with sub time resolution. The measured dynamics are nonexponential in time and are well described by a diffusioncontrolled electroncation recombination model. Excitationpowerdependent studies indicate that the unperturbed threephoton threshold ionization is only observed for pump irradiance below . At excitation fields above this level, the signal is no longer cubic in the excitation irradiance, and the observed electron survival probability dramatically changes, decaying as a single exponential in time.

Vibrational cooling in a cold ion trap: Vibrationally resolved photoelectron spectroscopy of cold anions
View Description Hide DescriptionWe demonstrate vibrational cooling of anions via collisions with a background gas in an ion trap attached to a cryogenically controlled cold head (10–400 K). Photoelectron spectra of vibrationally cold anions, produced by electrospray ionization and cooled in the cold ion trap, have been obtained. Relative to spectra taken at room temperature, vibrational hot bands are completely eliminated, yielding wellresolved vibrational structures and a more accurate electron affinity for neutral . The electron affinity of is measured to be . The cold spectra reveal complicated vibrational structures for the transition to the ground state due to the JahnTeller effect in the ground state of . Vibrational excitations in the two modes and eight modes are observed, providing ideal data to assess the vibronic couplings in

NMR shielding constants for hydrogen guest molecules in structure II clathrates
View Description Hide DescriptionProton NMR shielding constants and chemical shifts for hydrogen guests in small and large cages of structure II clathrates are calculated using densityfunctional theory and the gaugeinvariant atomicorbital method. Shielding constants are calculated at the B3LYP level with the basis set. The calculated chemical shifts are corrected with a linear regression to reproduce the experimental chemical shifts of a set of standard molecules. The calculated chemical shifts of single hydrogen molecules in the small and large structure II cages are 4.94 and 4.84 ppm, respectively, which show that within the error range of the method the guest molecules in the small and large cages cannot be distinguished. Chemical shifts are also calculated for double occupancy of the hydrogen guests in small cages, and double, triple, and quadruple occupancy in large cages. Multiple occupancy changes the chemical shift of the hydrogen guests by approximately 0.2 ppm. The relative effects of other guest molecules and the cage on the chemical shift are studied for the cages with multiple occupancies.

A new moleculardynamics based approach for molecular crystal structure search
View Description Hide DescriptionA new moleculardynamics based approach is proposed to search for candidate crystal structures of molecular solids. The procedure is based on the observation of spontaneous transitions between ordered and disordered states in moleculardynamics simulations of an artificial periodic system with a small unit cell. In such a way only the most stable structures are automatically selected. The method can be applied to the solution of crystal structures from lowquality or very complex diffraction data. Tests are presented for ice polymorphs.
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 ARTICLES

 Theoretical Methods and Algorithms

Coupledcluster methods including noniterative corrections for quadruple excitations
View Description Hide DescriptionA new method is presented for treating the effects of quadruple excitations in coupledcluster theory. In the approach, quadruple excitation contributions are computed from a formula based on a nonHermitian perturbation theory analogous to that used previously to justify the usual noniterative triples correction used in the coupled cluster singles and doubles method with a perturbative treatment of the triple excitations (CCSD(T)). The method discussed in this paper plays a parallel role in improving energies obtained with the full coupledcluster singles, doubles, and triples method (CCSDT) by adding a perturbative treatment of the quadruple excitations (CCSDT(Q)). The method is tested for an extensive set of examples, and is shown to provide total energies that compare favorably with those obtained with the full singles, doubles, triples, and quadruples (CCSDTQ) method.

Timedependent fourcomponent relativistic densityfunctional theory for excitation energies. II. The exchangecorrelation kernel
View Description Hide DescriptionWe extend our previous formulation of timedependent fourcomponent relativistic densityfunctional theory [J. Gao, W. Liu, B. Song, and C. Liu, J. Chem. Phys.121, 6658 (2004)] by using a noncollinear form for the exchangecorrelation kernel. The new formalism can deal with excited states involving moment (spin)flipped configurations which are otherwise not accessible with ordinary exchangecorrelation functionals. As a first application, the global potentialenergy curves of 16 lowlying coupled electronic states of the AuH molecule have been investigated. The derived spectroscopic parameters, including the adiabatic and vertical excitation energies, equilibrium bond lengths, harmonic and anharmonic vibrational constants, fundamental frequencies, and dissociation energies, are grossly in good agreement with those of ab initio multireference secondorder perturbation theory and the available experimental data.

Phasespace overlap measures. I. Failsafe bias detection in free energies calculated by molecular simulation
View Description Hide DescriptionWe consider ways to quantify the overlap of the parts of phase space important to two systems, labeled and . Of interest is how much of the important phase space lies in that important to , and how much of lies in . Two measures are proposed. The first considers four totalenergy distributions, formed from all combinations made by tabulating either the system or the system energy when sampling either the or system. Measures for in and in are given by two overlap integrals defined on pairs of these distributions. The second measure is based on information theory, and defines two relative entropies which are conveniently expressed in terms of the dissipated work for freeenergyperturbation (FEP) calculations in the and directions, respectively. Phasespace overlap is an important consideration in the performance of freeenergy calculations. To demonstrate this connection, we examine bias in FEP calculations applied to a system of independent particles in a harmonic potential. Systems are selected to represent a range of overlap situations, including extreme subset, subset, partial overlap, and nonoverlap. The magnitude and symmetry of the bias ( vs ) are shown to correlate well with the overlap, and consequently with the overlap measures. The relative entropies are used to scale the amount of sampling to obtain a universal bias curve. This result leads to develop a simple heuristic that can be applied to determine whether a workbased freeenergy measurement is free of bias. The heuristic is based in part on the measured free energy, but we argue that it is failsafe inasmuch as any bias in the measurement will not promote a false indication of accuracy.

Avoiding negative populations in explicit Poisson tauleaping
View Description Hide DescriptionThe explicit tauleaping procedure attempts to speed up the stochastic simulation of a chemically reacting system by approximating the number of firings of each reaction channel during a chosen time increment as a Poisson random variable. Since the Poisson random variable can have arbitrarily large sample values, there is always the possibility that this procedure will cause one or more reaction channels to fire so many times during that the population of some reactant species will be driven negative. Two recent papers have shown how that unacceptable occurrence can be avoided by replacing the Poisson random variables with binomial random variables, whose values are naturally bounded. This paper describes a modified Poisson tauleaping procedure that also avoids negative populations, but is easier to implement than the binomial procedure. The new Poisson procedure also introduces a second control parameter, whose value essentially dials the procedure from the original Poisson tauleaping at one extreme to the exact stochastic simulation algorithm at the other; therefore, the modified Poisson procedure will generally be more accurate than the original Poisson procedure.

Determination of the chemical potential using energybiased sampling
View Description Hide DescriptionAn energybiased method to evaluate ensemble averages requiring testparticle insertion is presented. The method is based on biasing the sampling within the subdomains of the testparticle configurational space with energies smaller than a given value freely assigned. These energy wells are located via unbiased random insertion over the whole configurational space and are sampled using the socalled HitandRun algorithm, which uniformly samples compact regions of any shape immersed in a space of arbitrary dimensions. Because the bias is defined in terms of the energy landscape it can be exactly corrected to obtain the unbiased distribution. The testparticle energy distribution is then combined with the Bennett relation for the evaluation of the chemical potential. We apply this protocol to a system with relatively small probability of lowenergy testparticle insertion, liquid argon at high density and low temperature, and show that the energybiased Bennett method is around five times more efficient than the standard Bennett method. A similar performance gain is observed in the reconstruction of the energy distribution.

A JacobiWilson description coupled to a blockDavidson algorithm: An efficient scheme to calculate highly excited vibrational levels
View Description Hide DescriptionWe present a new approach based on the blockDavidson scheme which provides eigenvalues and eigenvectors of highly excited (ro) vibrational states of polyatomic molecules. The key ingredient is a prediagonalizedperturbative scheme applied to a subspace of a curvilinear normalmode basis set. This approach is coupled to the Jacobi vector description recently developed by our group [C. Leforestier, A. Viel, F. Gatti, C. Muñoz, and C. Iung, J. Chem. Phys.114, 2099 (2001)], and applied to the HFCO and molecules, which represent the main difficulties of such calculations for any available method. The first one presents a significant state density because of its low symmetry and the presence of a fluorine atom, while strong resonances and intermode couplings occur in . This study establishes the robustness, the numerical efficiency, and the versatility of the method which is compared to the regular Lanczos and Davidson schemes. It is also shown that the eigenvectors can be obtained within a given accuracy easily set by the user. This point constitutes one of the main advantages of the method as very few potentialenergysurfaces achieve an accuracy of the order of a wave number for highly excited states. Furthermore, this method allows one to restrict the calculations to selectedenergy levels based on their zeroorder descriptions.

Quantum hydrodynamics: Capturing a reactive scattering resonance
View Description Hide DescriptionThe hydrodynamicequations of motion associated with the de BroglieBohm formulation of quantum mechanics are solved using a meshless method based upon a moving leastsquares approach. An arbitrary LagrangianEulerian frame of reference and a regridding algorithm which adds and deletes computational points are used to maintain a uniform and nearly constant interparticle spacing. The methodology also uses averaged fields to maintain unitary time evolution. The numerical instabilities associated with the formation of nodes in the reflected portion of the wave packet are avoided by adding artificial viscosity to the equations of motion. A new and more robust artificial viscosity algorithm is presented which gives accurate scattering results and is capable of capturing quantum resonances. The methodology is applied to a onedimensional model chemical reaction that is known to exhibit a quantum resonance. The correlation function approach is used to compute the reactive scattering matrix, reaction probability, and time delay as a function of energy. Excellent agreement is obtained between the scattering results based upon the quantum hydrodynamic approach and those based upon standard quantum mechanics. This is the first clear demonstration of the ability of moving grid approaches to accurately and robustly reproduce resonance structures in a scattering system.

Quantuminstanton evaluation of the kinetic isotope effects
View Description Hide DescriptionA general quantummechanical method for computing kinetic isotope effects is presented. The method is based on the quantuminstanton approximation for the rate constant and on the pathintegral Metropolis–Monte Carlo evaluation of the Boltzmann operator matrix elements. It computes the kinetic isotope effect directly, using a thermodynamic integration with respect to the mass of the isotope, thus avoiding the more computationally expensive process of computing the individual rate constants. The method should be more accurate than variational transitionstate theories or the semiclassical instanton method since it does not assume a single tunneling path and does not use a semiclassical approximation of the Boltzmann operator. While the general Monte Carlo implementation makes the method accessible to systems with a large number of atoms, we present numerical results for the Eckart barrier and for the collinear and full threedimensional isotope variants of the hydrogen exchange reaction . In all seven test cases, for temperatures between 250 and 600 K, the error of the quantum instanton approximation for the kinetic isotope effects is less than .
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Photodissociation of HI and DI: Testing models for electronic structure via polarization of atomic photofragments
View Description Hide DescriptionThe photodissociation dynamics of HI and DI are examined using timedependent wavepacket techniques. The orientation and alignment parameters are determined as a function of photolysis energy for the resulting groundstate and excitedstate atoms. The parameters describe the coherent and incoherent contributions to the angular momentum distributions from the , , and electronic states accessed by perpendicular excitation and the state accessed by a parallel transition. The outcomes of the dynamics based on both shifted ab initio results and three empirical models for the potentialenergy curves and transitiondipole moments are compared and contrasted. It is demonstrated that experimental measurement of the parameters for the excitation from the vibrational ground state would be able to distinguish between the available models for the HI potentialenergy curves and transitiondipole moments. The differences between the parameters for the excitation from stand in sharp contrast to the scalar properties, i.e., total cross section and branching fraction, which require experimental measurement of photodissociation from excited vibrational states to distinguish between the models.

Highlevel ab initio studies of the structure, vibrational spectra, and energetics of
View Description Hide DescriptionObservation of massdependent and nonmassdependent sulfur isotope fractionations in elemental sulfur is providing new insight into the nature of the sulfur cycle in the atmosphere. Interpretation of the experimental isotope data requires estimation of the energetics for the reaction (isoelectronic with ). Key molecular properties of the potentialenergy surface, such as vibrational frequencies and isotopic shifts, are presented that can be used to assess the massdependent fractionation effect. Ab initio results are compared to the available experimental results for to evaluate the reliability of the computational results for . The S–S bonddissociation energy for is determined to be .

Experimental and modeling study of the ionmolecule association reaction
View Description Hide DescriptionExperimental results for the rate of the association reaction obtained with the Cinétique de Réactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases and , over the temperature range of , and at pressures between 0.16 and . At the highest temperatures, the reaction was found to be close to the limiting lowpressure termolecular range, whereas the limiting highpressure bimolecular range was approached at the lowest temperatures. Whereas the lowpressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived highpressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple iondipole capture theory. This result differs from previous observations on the related reaction . This observation is tentatively attributed to more pronounced contributions of the valence part of the potentialenergy surface to the reaction in than in . Falloff curves of the reaction are constructed over wide ranges of conditions and represented in compact analytical form.

Spectroscopic and theoretical characterization of the transition of CH–Ne
View Description Hide DescriptionThe transition of CH–Ne was examined using laserinduced fluorescence and fluorescence depletion techniques. The spectrum was found to be particularly congested due to the large number of bound states derived from the interaction, and the small energy spacings between these states resulting from the relatively weak anisotropy of the van der Waals bond. Highlevel ab initio calculations were used to generate twodimensional potential energy surfaces for and . The equilibrium structures from these surfaces were bent and linear for the and states, respectively. Variational calculations were used to predict the bound states supported by the ab initiosurfaces. Empirical modification of the potential energy surfaces for the state was used to obtain energylevel predictions that were in good agreement with the experimental results. Transitions to all of the optically accessible internal rotor states of were identified, indicating that CH performs hindered internal rotations in the lowestenergy levels of the and states. The characteristics of the potential energy surfaces for CH–Ne in the , and states suggest that dispersion and exchange repulsion forces dominate the van der Waals interaction.