Volume 123, Issue 17, 01 November 2005
 COMMUNICATIONS


Observation of fragiletostrong liquid transition in surface water in
View Description Hide DescriptionA quasielastic neutronscattering experiment carried out on a backscattering spectrometer with sub resolution in the temperature range of has revealed the dynamics of surface water in cerium oxide on the time scale of hundreds of picoseconds. This slow dynamics is attributed to the translational mobility of the water molecules in contact with the surface hydroxyl groups. The relaxation function of this slow motion can be described by a slightly stretched exponential with the stretch factor exceeding 0.9, which indicates almost a Debyetype dynamics. Down to about , the temperature dependence of the residence time for water molecules follows a VogelFulcherTamman law with the glass transition temperature of . At lower temperatures, the residence time behavior abruptly changes, indicating a fragiletostrong liquid transition in surface water at about .
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 ARTICLES

 Theoretical Methods and Algorithms

Energy band gaps and lattice parameters evaluated with the HeydScuseriaErnzerhof screened hybrid functional
View Description Hide DescriptionThis work assesses the HeydScuseriaErnzerhof (HSE) screened Coulomb hybrid density functional for the prediction of lattice constants and band gaps using a set of 40 simple and binary semiconductors. An extensive analysis of both basis set and relativistic effects is given. Results are compared with established pure density functionals. For lattice constants, HSE outperforms local spindensity approximation (LSDA) with a mean absolute error (MAE) of 0.037 Å for HSE vs 0.047 Å for LSDA. For this specific test set, all pure functionals tested produce MAEs for band gaps of 1.0–1.3 eV, consistent with the very wellknown fact that pure functionals severely underestimate this property. On the other hand, HSE yields a MAE smaller than 0.3 eV. Importantly, HSE correctly predicts semiconducting behavior in systems where pure functionals erroneously predict a metal, such as, for instance, Ge. The shortrange nature of the exchange integrals involved in HSE calculations makes their computation notably faster than regular hybrid functionals. The current results, paired with earlier work, suggest that HSE is a fast and accurate alternative to established density functionals, especially for solid state calculations.

Densityfunctional calculations of relativistic spinorbit effects on nuclear magnetic shielding in paramagnetic molecules
View Description Hide DescriptionTerms arising from the relativistic spinorbit effect on both hyperfine and Zeemaninteractions are introduced to densityfunctional theory calculation of nuclear magnetic shielding in paramagnetic molecules. The theory is a generalization of the former nonrelativistic formulation for doublet systems and is consistent to , the fourth power of the fine structure constant, for the spinorbit terms. The new temperaturedependent terms arise from the deviation of the electronic tensor from the freeelectron value as well as spinorbit corrections to hyperfine coupling tensor, the latter introduced in the present work. In particular, the new contributions include a redefined isotropic pseudocontact contribution that consists of effects due to both the tensor and spinorbit corrections to hyperfine coupling. The implementation of the spinorbit terms makes use of allelectron atomic meanfield operators and/or spinorbit pseudopotentials. Sample results are given for group9 metallocenes and a nitroxide radical. The new corrections are found significant for the metallocene systems while they obtain small values for the nitroxide radical. For the isotropic shifts, none of the three beyondleadingorder hyperfine contributions are negligible.

Nucleationcontrolled multiphase transitions
View Description Hide DescriptionThermodynamics and kinetics of phase transitions in multiphase systems have been discussed. Thermodynamically admissible transitions have been identified and transition kinetics described in terms of the extended KolmogoroffAvramiEvans model. Different combinations of transitions have been described as directed graphs. Graph nodes represented individual phases, graph edges—transitions. Superposition of parallel transitions in various mother phases, simultaneous transition of the same mother phase into different target phases, and successive (chain) transitions have been analyzed. Detailed solutions for a threephase system consisting of one liquid phase and two polymorphic solid phases have been presented.

Orbitalfree embedding applied to the calculation of induced dipole moments in (, Ne, Ar, Kr, Xe, Hg) van der Waals complexes
View Description Hide DescriptionThe orbitalfree frozendensity embedding scheme within densityfunctional theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem.97, 8050 (1993)] is applied to the calculation of induced dipole moments of the van der Waals complexes (, Ne, Ar, Kr, Xe, Hg). The accuracy of the embedding scheme is investigated by comparing to the results of supermolecule KohnSham densityfunctional theory calculations. The influence of the basis set and the consequences of using orbitaldependent approximations to the exchangecorrelation potential in embedding calculations are examined. It is found that in supermolecular KohnSham densityfunctional calculations, different common approximations to the exchangecorrelation potential are not able to describe the induced dipole moments correctly and the reasons for this failure are analyzed. It is shown that the orbitalfree embedding scheme is a useful tool for applying different approximations to the exchangecorrelation potential in different subsystems and that a physically guided choice of approximations for the different subsystems improves the calculated dipole moments significantly.

Calculation of specific, highly excited vibrational states based on a Davidson scheme: Application to HFCO
View Description Hide DescriptionWe present the efficiency of a new modified Davidson scheme which yields selectively one highenergy vibrationally excited eigenstate or a series of eigenstates. The calculation of a highly vibrationally excited state located in a dense part of the spectrum requires a specific prediagonalization step before the Davidson scheme. It consists in building a small active space containing the zeroorder states which are coupled with the zeroorder description of the eigenstate of interest. We propose a general way to define this active space which plays a crucial role in the method. The efficiency of the method is illustrated by computing and analyzing the highenergy excited overtones of the outofplane mode in HFCO. These overtone energies correspond to the 234th, 713th, and 1774th energy levels in our reference basis set which contains roughly 140 000 states. One of the main advantages of this Davidson scheme comes from the fact that the eigenstate and eigenvalue convergence can be assessed during the iterations by looking at the residual . The maximum value allowed for this residual constitutes a very sensitive and efficient parameter which sets the accuracy of the eigenvalues and eigenstates, even when the studied states are highly excited and are localized in a dense part of the spectrum. The physical analysis of the eigenstates associated with the 5th, 7th, and 9th outofplane overtones in HFCO provides some interesting information on the energy localization in this mode and on the role played by the inplane modes. Also, it provides some ideas on the numerical methods which should be developed in the future to tackle higherenergy excited states in polyatomics.

Coherent excitation of a twostate system by a linearly chirped Gaussian pulse
View Description Hide DescriptionThis work presents an analytic description of coherent excitation of a twostate quantum system by an external field with a Gaussian temporal shape and a linear frequency sweep. A very accurate analytic approximation to the transition probability in terms of the Lambert function is derived by using the DykhneDavisPechukas approach. This approximation provides analytic expressions for the frequency and the amplitude of the probability oscillations and for the ranges of interaction parameters where high transition probability is obtained.

Transition probabilities of a string oscillator subject to impulsive collisions with a heavy mass point
View Description Hide DescriptionImpulsive linear collisions between a string oscillator (a onedimensional particle in a box) and a mass point are studied quantum mechanically. In the limit of a very heavy mass point (which corresponds classically to many collisions during a single encounter) the transition probabilities are determined exactly. The result permits a discussion of the mixed quantumclassical regime where the collider becomes almost classical while the oscillator remains quantum mechanical. While the average transition probabilities are well reproduced by the Ehrenfest meanfield approximation, the prediction for the superimposed highfrequency resonance structure is qualitatively wrong for a genuine quantum oscillator. Only if the oscillator is also almost classical and if , where is the mass ratio collider∕oscillator, this structure is correctly predicted by the Ehrenfest approximation.

Semiclassical nonadiabatic dynamics using a mixed wavefunction representation
View Description Hide DescriptionNonadiabaticeffects in quantum dynamics are described using a mixed polar/coordinate space representation of the wave function. The polar part evolves on dynamically determined potential surfaces that have diabatic and adiabatic potentials as limiting cases of weak localized and strong extended diabatic couplings. The coordinate space part, generalized to a matrix form, describes transitions between the surfaces. Choice of the effective potentials for the polar part and partitioning of the wave function enables one to represent the total wave function in terms of smooth components that can be accurately propagated semiclassically using the approximate quantum potential and small basis sets. Examples are given for twostate onedimensional problems that modelchemical reactions that demonstrate the capabilities of the method for various regimes of nonadiabaticdynamics.

Pair potentials from diffraction data on liquids: A neural network solution
View Description Hide DescriptionThe inverse theorem of liquids states a one to one correspondence between classical mechanical pair potentials and structural functions. Moleculardynamics and Monte Carlo simulations provide exact structural functions for known pair interactions. There is no exact or widespread method in the opposite direction, where the pair interactions are to be determined from a priori known paircorrelation functions or structure factors. The methods based on the integral equation theories of liquids are approximate and the iterative refinements of pair potentials with simulations take a long time. We applied artificial neural networks to get pair interactions from known structure factors in this study. We performed moleculardynamics simulations on onecomponent systems with different pair potentials and the structure factors were calculated. To optimize (train) the weights of neural networks 2000 pair interactionstructure factor pairs were used. The performance of the method was tested on further 200 data pairs. The method provided reasonable potentials for the majority of the systems opening a “quick and dirty” method to determine pair interactions.

Theory and method for calculating resonance Raman scattering from resonance polarizability derivatives
View Description Hide DescriptionWe present a method to calculate both normal Ramanscattering (NRS) and resonance Ramanscattering (RRS) spectra from the geometrical derivatives of the frequencydependent polarizability. In the RRS case, the polarizability derivatives are calculated from resonance polarizabilities by including a finite lifetime of the electronic excited states using timedependent densityfunctional theory. The method is a shorttime approximation to the Kramers, Heisenberg, and Dirac formalism. It is similar to the simple excitedstategradient approximation method if only one electronic excited state is important, however, it is not restricted to only one electronic excited state. Since the method can be applied to both NRS and RRS, it can be used to obtain complete Raman excitation profiles. To test the method we present the results for the state of uracil and the , and states of pyrene. As expected, the results are almost identical to the results obtained from the excitedstategradient approximation method. Comparing with the experimental results, we find in general quite good agreement which enables an assignment of the experimental bands to bands in the calculated spectrum. For uracil the inclusion of explicit waters in the calculations was found to be necessary to match the solution spectra. The calculated resonance enhancements are on the order of , which is in agreement with experimental findings. For pyrene the method is also able to distinguish between the three different electronic states for which experimental data are available. The neglect of anharmonicity and solvent effects in the calculations leads to some discrepancy between theory and experiment.

Information entropy, information distances, and complexity in atoms
View Description Hide DescriptionShannon informationentropies in position and momentum spaces and their sum are calculated as functions of in atoms. RoothaanHartreeFock electron wave functions are used. The universal property is verified. In addition, we calculate the KullbackLeibler relative entropy, the JensenShannon divergence, Onicescu’s informationenergy, and a complexity measure recently proposed. Shell effects at closedshell atoms are observed. The complexity measure shows local minima at the closedshell atoms indicating that for the above atoms complexity decreases with respect to neighboring atoms. It is seen that complexity fluctuates around an average value, indicating that the atom cannot grow in complexity as increases. Onicescu’s informationenergy is correlated with the ionization potential. Kullback distance and JensenShannon distance are employed to compare RoothaanHartreeFock density distributions with other densities of previous works.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Isomers of : IRabsorption spectra of ONCO in solid Ne
View Description Hide DescriptionIrradiation of a Ne matrix sample containing NO and CO near with an ArF excimer laser at yielded new lines at 2045.1 and that were depleted upon secondary photolysis at . These lines are assigned to stretching and mixed stretching modes of ONCO, based on results of , , and isotopic experiments and quantumchemical calculations. These calculations using densityfunctional theory (B3LYP and PW91PW91/augccpVTZ) predict five stable isomers of : ONCO, NCOO, , CNOO, and cycCNOO, listed in order of increasing energy. According to B3LYP calculations, ONCO has a trans configuration, with bond angles of and . Calculated vibrational wave numbers, IR intensities, , , and isotopic shifts for ONCO agree satisfactorily with experimental results. ONCO was formed from reaction of CO with NO in its excited state.

The vibrational energy pattern in acetylene VII:
View Description Hide DescriptionIn 129 vibrational term values up to are merged, about 60% of which are newly reported. They are fitted using an effective Hamiltonian with a standard deviation of . The vibrational assignments and vibrational constants are listed and discussed. The energy pattern is found to be very similar to the one in with additional anharmonic resonances arising from the lack of character in the asymmetric isotopolog.

The UV photodissociation dynamics of ClO radical using velocity map ion imaging
View Description Hide DescriptionWe have studied the wavelengthdependent photodissociation dynamics of jetcooled ClO radical from using velocity map ion imaging. We find that is the dominant channel above the threshold with minor contributions from the and channels. We have measured the photofragment angular distributions for each dissociation channel and find that the state reached via a parallel transition carries most of the oscillator strength above the threshold. The formation of fragments with positive anisotropy is evidence of curve crossing from the state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the threshold to be by measuring quantum yield in the wavelength range of . We also report on the predissociation dynamics of ClO below the threshold. We find that the branching ratio of is at both 266 and . The rotational depolarization of the anisotropy parameters of the fragments provides predissociation lifetimes of for the 90 band and for the 80 band, in reasonable agreement with previous spectroscopic and theoretical studies.

Ab initio relativistic calculation of the RbCs molecule
View Description Hide DescriptionWe apply the relativistic configurationinteraction valencebond method to calculate various characteristics of the alkalimetal RbCs dimer. These include the electronic potentials and transitiondipole moments between the ground and first excited states and permanent dipole moments of the and states of the ground configuration. In addition, we estimate the lifetime of the rovibrational levels of the state due to blackbody radiation. These data can help experimentalists to optimize photoassociative formation of ultracold RbCs molecules and their longevity in a trap or in an optical lattice. Extended basis sets, constructed from DiracFock and Sturm’s orbitals, have been used to ensure convergence of our calculations. We compare our data with other theoretical and experimental results when they were available.

: Ab initio anharmonic force field, dipole moments, and infrared vibrational transitions
View Description Hide DescriptionThe vibrational spectra of have been recorded in the region, utilizing the Fouriertransform spectroscopy and Fouriertransform intracavity laser absorption spectroscopy. Totally 61 band centers and intensities are derived from the infrared spectra. An ab initio quartic force field is obtained by applying the secondorder MøllerPlesset perturbation theory and correlationconsistent polarized valence tripletzeta basis sets [J. Chem. Phys.90, 1007 (1989); 98, 1358 (1993)]. Most observed bands are assigned by the vibration analysis based on the secondorder perturbation theory. Reduceddimensional ab initiodipole moment functions (two dimensional and three dimensional) have also been calculated to investigate the absolute band intensities of the chromophore. The calculated values agree reasonably with the observed ones.

Stateresolved collisional quenching of vibrationally excited pyrazine by
View Description Hide DescriptionSupercollision relaxation of highly vibrationally excited pyrazine with is investigated using highresolution transient IR diode laserabsorption spectroscopy at . Highly excited pyrazine is prepared by pulsed UV excitation at , followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl molecules with is characterized by . The relative translational energy increases as a function of rotational quantum number for DCl with for and for . The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energytransfer rate for appearance of DCl is , approximately oneeighth the LennardJones collision rate. The results are compared to earlier energy gain measurements of and .

Lifetime and yield of metastable ions
View Description Hide Descriptionions produced in a cooled supersonic expansion by electronimpact ionization are accelerated at 2.5 keV and kept during few milliseconds inside a linear electrostatic trap. The lifetime of the metastable ion is determined from the measurement of the rate of the argon atoms escaping the trap. The lifetime and the relative metastable populations are measured as a function of the pressure and temperature in the supersonic expansion, i.e., of the mean cluster size. Possible mechanisms responsible for the metastable formation are discussed.

Threeisotope plot of fractionation in photolysis: A perturbation theoretical expression
View Description Hide DescriptionThe slope of the threeisotope plot for the isotopomer fractionation by direct or nearly direct photodissociation is obtained using a perturbationtheoretical analysis. This result, correct to first order in the mass difference, is the same as that for equilibrium chemical exchange reactions, a similarity unexpected a priori. A comparison is made with computational results for photodissociation. This theoretical slope for massdependent photolytic fractionation can be used to analyze the data for isotopic anomalies in spinallowed photodissociationreactions. Earlier work on chemical equilibria is extended by avoiding a hightemperature approximation.