Volume 130, Issue 22, 14 June 2009

Recently, the supercooled Wahnström binary LennardJones mixture was partially crystallized into phase crystals in lengthy molecular dynamics simulations. We present molecular dynamics simulations of a modified Kob–Andersen binary LennardJones mixture that also crystallizes in lengthy simulations here, however, by forming pure fcc crystals of the majority component. The two findings motivate this paper that gives a general thermodynamic and kinetic treatment of the stability of supercooled binary mixtures, emphasizing the importance of negative mixing enthalpy whenever present. The theory is used to estimate the crystallization time in a Kob–Andersen mixture from the crystallization time in a series of related systems. At we estimate this time to be time units . A new binary LennardJones mixture is proposed that is not prone to crystallization and faster to simulate than the two standard binary LennardJones mixtures. This is obtained by removing the likeparticle attractions by switching to Weeks–Chandler–Andersen type potentials, while maintaining the unlikeparticle attraction.
 ARTICLES

 Theoretical Methods and Algorithms

Time propagation of the Kadanoff–Baym equations for inhomogeneous systems
View Description Hide DescriptionWe have developed a timepropagation scheme for the Kadanoff–Baym equations for general inhomogeneous systems. These equations describe the time evolution of the nonequilibrium Green function for interacting manybody systems in the presence of timedependent external fields. The external fields are treated nonperturbatively whereas the manybody interactions are incorporated perturbatively using derivable selfenergy approximations that guarantee the satisfaction of the macroscopic conservation laws of the system. These approximations are discussed in detail for the timedependent Hartree–Fock, the second Born, and the approximation.

Convexset description of quantum phase transitions in the transverse Ising model using reduceddensitymatrix theory
View Description Hide DescriptionQuantum phase transitions in particle systems can be identified and characterized by the movement of the twoparticle reduced density matrix (2RDM) along the boundary of its representable convex set as a function of the Hamiltonian parameter controlling the phase transition [G. Gidofalvi and D. A. Mazziotti, Phys. Rev. A74, 012501 (2006)]. For the onedimensional transverse Ising modelquantum phase transitions as well as their finitelattice analogs are computed and characterized by the 2RDM movement with respect to the transverse magnetic field strength . The definition of a 2RDM “speed” quantifies the movement of the 2RDM per unit of , which reaches its maximum at the critical point of the phase transition. For the infinite lattice the convex set of 2RDMs and the 2RDM speed are computed from the exact solution of the 2RDM in the thermodynamic limit of infinite [P. Pfeuty, Ann. Phys.57, 79 (1970)]. For the finite lattices we compute the 2RDM convex set and its speed by the variational 2RDM method [D. A. Mazziotti, Phys. Rev. Lett.93, 213001 (2004)] in which approximate groundstate 2RDMs are calculated without particle wave functions by using constraints, known as representability conditions, to restrict the 2RDMs to represent quantum system of fermions. Advantages of the method include: (i) rigorous lower bounds on the groundstateenergies, (ii) polynomial scaling of the calculation with , and (iii) independence of the representability conditions from a reference wave function, which enables the modeling of multiple quantum phases. Comparing the 2RDM convex sets for the finite and infinitesite lattices reveals that the variational 2RDM method accurately captures the shape of the convex set and the signature of the phase transition in the 2RDM movement. From the 2RDM all one and twoparticle expectation values (or order parameters) of the quantum Ising model can also be computed including the pair correlation function, which decays rapidly around the critical field strength .

Multicomponent gauge cell method
View Description Hide DescriptionThe gauge cell Monte Carlo method [Neimark and Vishnyakov, J. Chem. Phys.122, 234108 (2005)] for calculations of chemical potential in dense and strongly inhomogeneous fluids is extended to multicomponent systems. The system of interest is simulated in a sample cell that is placed in chemical contact with several gauge cells of limited capacity, one gauge cell per component. Thus, each component can be exchanged between the sample cell and the respective gauge cell. The sample and gauge cells are immersed into the thermal bath of a given temperature. The size of the gauge cell controls the level of concentration fluctuations for the respective component in the sample cell. The chemical potentials are rigorously calculated from the equilibrium distribution of particles between the system and the gauges, and the results do not depend on the gauge size. For large systems, the chemical potentials can be accurately estimated from the average densities in the gauge cells. The proposed method was tested against the literature data on the vaporliquid equilibrium in a binary mixture of subcritical and supercritical fluids and against the grand canonical and Widom insertion Monte Carlo methods for a binary mixture confined to a very narrow spherical pore. The method is specifically suitable for simulations of metastable and labile states in multicomponent confined fluids.

Virial coefficients of LennardJones mixtures
View Description Hide DescriptionWe report results of calculations of the second through sixth virial coefficients for four prototype LennardJones (LJ) mixtures that have been the subject of previous studies in the literature. Values are reported for temperatures ranging from to , where here the temperature is given units of the LJ energy parameter of one of the components. Thermodynamic stability of the mixtures is studied using the virial equation of state (VEOS) with the calculated coefficients, with particular focus on characterizing the vaporliquid critical behavior of the mixtures. For three of the mixtures, vaporliquid coexistence and critical data are available for comparison at only one temperature, while for the fourth we can compare to a critical line. We find that the VEOS provides a useful indication of the presence and location of critical behavior, although in some situations we find need to consider “nearmiss” critical behavior, where the classical conditions of criticality are nearly but not exactly satisfied.

Calculation of rovibronic intensities for triatomic molecules in doubleRennerdegenerate electronic states: Application to the and electronic states of
View Description Hide DescriptionAn algorithm and a computer program implementing it are presented for calculation of the rovibronic intensities for a triatomic molecule in a “doubleRennerdegenerate” electronic state. The program has been applied to investigate, by theoretical simulation, the absorptionspectrum of in the and electronic states. The spectrum simulations are based on potential energy functions, electric dipole moment functions, and electric dipole transition moment functions constructed from ab initio values calculated at the corevalence /[ccpVQZ (H), augccpCVQZ (O)] level of theory.

Liouville–von Neumann molecular dynamics
View Description Hide DescriptionWe present a novel first principles molecular dynamics scheme, called Liouville–von Neumann molecular dynamics, based on Liouville–von Neumann equation for density matrices propagation and Magnus expansion of the timeevolution operator. The scheme combines formally accurate quantum propagation of electrons represented via density matrices and a classical propagation of nuclei. The method requires a few iterations per each time step where the Fock operator is formed and von Neumann equation is integrated. The algorithm (a) is free of constraint and fictitious parameters, (b) avoids diagonalization of the Fock operator, and (c) can be used in the case of fractional occupation as in metallic systems. The algorithm is very stable, and has a very good conservation of energy even in cases when a good quality conventional Born–Oppenheimer molecular dynamics trajectories is difficult to obtain. Test simulations include initial phase of fullerene formation from gaseous and retinal system.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Theoretical and kinetic studies of the reactions of and with hydroxyl radicals
View Description Hide DescriptionThe hydrogen abstraction reactions of fluoroalkane isomers and with the OH radicals have been studied theoretically by a duallevel direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimumenergy path are obtained at the level of theory, and then the energy profiles are refined at G3(MP2) level of theory. Using the improved canonical variational transitionstate theory (ICVT) with the smallcurvature tunneling correction (SCT), the rate constants for each channel are calculated over a wide temperature range of 200–1000 K. Our results show that the tunneling correction plays an important role in the rate constant calculation in the low temperature range. The calculated ICVT/SCT rate constants are consistent with available experimental data. Our calculations indicate the contribution of the abstraction from the –CFH– group of isomeric compounds and to the overall reactions is quite different over the whole temperature range due to the effect of different groups at both sides of –CFH– group. Furthermore, to further reveal the thermodynamic properties, the enthalpies of formation of the two reactants , and , and the product radicals , , , and are obtained by using isodesmic reactions.

Density functional theory study of multiply ionized weakly bound fullerene dimers
View Description Hide DescriptionMultiply ionized fullerene dimers have been studied by means of stateoftheart density functional theory methods. We found that the singly charged dimer is more strongly bound than the corresponding neutral van der Waals dimer at the binding distance of the latter; in contrast, multiply charged dimers are unstable. For the latter dimers, the calculated kinetic energy release in the binary fission yielding intact fullerenes is lower than those reported in recent experimental work. This implies that, in such experiments, there are significant internal excitations in the separating monomers. We also show that electron transfer within the charged dimers occurs on the subfemtosecond time scale, in accordance with the high charge mobility observed in dimers and larger clusters of fullerenes. This provides an explanation for the evenodd effects in the measured multiple ionization yields of fullerene dimers.

The structure of phenol clusters in their and states
View Description Hide DescriptionThe structures of the van der Waals bonded complexes of phenol with one and two argon atoms have been determined using rotationally resolved electronic spectroscopy of the transition. The experimentally determined structural parameters were compared to the results of quantum chemical calculations that are capable of properly describing dispersive interactions in the clusters. It was found that both complexes have bound configurations, with the phenol complex adopting a symmetric structure. The distances of the argon atoms to the aromatic plane in the electronic ground state of the and clusters are 353 and 355 pm, respectively. Resonanceenhanced multiphoton ionization spectroscopy was used to measure intermolecular vibrational frequencies in the state and Franck–Condon simulations were performed to confirm the structure of the phenol cluster. These were found to be in excellent agreement with the configuration.

Fourier transform microwave spectroscopy of
View Description Hide DescriptionRotational spectra of the radical in the ground state with resolved fine and hyperfine structures have been observed for the first time by Fourier transform microwave spectroscopy. Rotational transitions are analyzed using the ordinary rigid asymmetric top Hamiltonian for doublet species with three nuclear spins. Seventeen molecular constants including the fine and hyperfine coupling constants have been determined. The radical is confirmed to be a planer molecule in the ground electronic state since it has a small positive inertial defect. From the hyperfine coupling constants of protons, the unpaired electron density on the carbon atom is estimated to be about 85%. Thus, the formylmethyltype resonance form is a dominant structure in the ground electronic state, and fluorine substitution has negligible effect on the C–C–O conjugate system.

A dose dependence study of adsorbed on large Ar clusters
View Description Hide DescriptionAn investigation of the behavior of molecules in and on doped large Ar host clusters has been performed by means of core and valence photoelectron spectroscopy. Data from pure and Ar clusters, as well as from doped Ar clusters, are presented. The experimental data together with calculations of the binding energy shifts of oxygen molecular ions in and on the surface of a large host Ar cluster show that the diffusion behavior has a strong dependence on the dopingpressure. We conclude that the oxygen molecules in the doped Ar host do not partake in band formation, since there is clear vibrational resolution in the spectral features stemming from screened ions. This implies that valence photoelectron spectroscopy can be used to determine the geometricalstructure of this and certain, similar, cluster systems.

Coupled translationrotation eigenstates of in and on the spectroscopically optimized interaction potential: Effects of cage anisotropy on the energy level structure and assignments
View Description Hide DescriptionWe have developed a quantitatively accurate pairwise additive fivedimensional (5D) potential energy surface (PES) for in through fitting to the recently published infrared (IR) spectroscopic measurements of this system for in the vibrationally excited state. The PES is based on the threesite pair potential introduced in this work, which in addition to the usual LennardJones (LJ) interaction sites on each H atom of has the third LJ interaction site located at the midpoint of the H–H bond. For the optimal values of the three adjustable parameters of the potential model, the fully coupled quantum 5D calculations on this additive PES reproduce the six translationrotation (TR) energy levels observed so far in the IR spectra of to within 0.6%. This is due in large part to the greatly improved description of the angular anisotropy of the fullerene interaction afforded by the threesite pair potential. The same pair potential spectroscopically optimized for was also used to construct the pairwise additive 5D PES of in . This PES, because of the lower symmetry of relative to that of , exhibits pronounced anisotropy with respect to the direction of the translational motion of away from the cage center, unlike that of in . As a result, the TR energy level structure of in from the quantum 5D calculations on the optimized PES, the quantum numbers required for its assignment, and the degeneracy patterns which arise from the TR coupling for translationally excited are all qualitatively different from those determined previously for [M. Xu et al. , J. Chem. Phys.128, 011101 (2008).

Spectroscopic study on the structural isomers of and multipleproton transfer reactions in the gas phase
View Description Hide DescriptionThe resonanceenhanced twophoton ionization (RE2PI) and laserinduced fluorescence excitation spectra were recorded for the region of the [ ] clusters in the gas phase to investigate the geometrical structures and the multipleproton/hydrogen atom transfer reaction dynamics. Four and two structural isomers were identified for and , respectively. Density functional theory calculations at the level predicted four different conformations of the ethyl group for , in good agreement with the observation of the four structural isomers in the RE2PI spectra. Visible fluorescence from the tautomeric forms was observed in the states for all isomers of , but no sign of doubleproton/hydrogen atom transfer and quadrupleproton/hydrogen atom transfer has been obtained in the electronic spectra of and , respectively. These results suggest that the multipleproton transfer reaction is clustersize selective, and the tripleproton/hydrogen atom transfer potential is dominated by the cyclic hydrogenbonded network in . The excitation of the inphase intermolecular stretching vibration prominently enhances the excitedstate tripleproton/hydrogen atom transfer reaction.

Heat capacities of freely evaporating charged water clusters
View Description Hide DescriptionWe report on evaporation studies on positively chargedwater clusters and negatively charged mixed clusters with a small core ion (, , or ), in the size range . The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters’ heat capacities and this allowed the determination of sizedependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of per added molecule. For clusters with the heat capacities per molecule are lower but the incremental increase higher. For the values are intermediate between the bulk liquid and the solid water values.

Formation and photodestruction of dual dipolebound anion
View Description Hide DescriptionA new type of dipolebound anion composed of water and nitromethane is formed via the incorporation of into argonsolvated water hexamer anions, . The reaction proceeds as an Armediated process such that an effective energy dissipation through sequential Ar evaporation gives rise to the formation of . Photoelectron spectroscopy is employed to probe the electronic properties of the anion, which reveals that the dipolebound nature of remains almost intact in the product anion; the vertical detachment energy of is determined to be . This spectroscopic finding, together with other suggestive evidences, allows us to refer to as a dual dipolebound anion described as , where the diffuse excess electron interacts with both the and moieties via the electrondipole interactions. The photodestruction of at 2134 nm (0.58 eV) occurs with a competition between electron detachment and fragmentation. The latter leads exclusively to the formation of , indicating that the dual dipolebound anion serves as a precursor to the hydrated valence anion of .

Infrared spectra of the complex: Observation of two distinct slipped nearparallel isomers
View Description Hide DescriptionInfrared spectra of complexes are studied in a pulsed supersonic slitjet expansion using a tunable diode laser probe in the region of the C–O stretching fundamental of OCS. Two bands are observed and analyzed, corresponding to two distinct isomers of the complex. Isomer a is the known form which has been previously studied in the microwave region. Isomer b is a new form, expected theoretically but first observed here. Structures are determined with the help of isotopic substitution. Both isomers are planar, with slipped nearparallel geometries. In isomer a, the intermolecular (center of mass) separation is 3.55 Å and the C atom of the is closer to the S atom of the OCS. In isomer b, the C atom of slides closer to the O atom of OCS and the center of mass separation increases to 3.99 Å. Isomer a is the lowest energy form, but paradoxically isomer b appears to be stronger in our infrared spectra. Predicted pure rotational transition frequencies are given to help in a search for the microwave spectrum of isomer b.

A new potential energy surface and predicted infrared spectra of the van der Waals complex
View Description Hide DescriptionA new potential energy surface for is constructed at the coupledcluster singles and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlationconsistent triplezeta (augccpVTZ) basis set plus midpoint bond functions. The normal mode for the antisymmetric stretching vibration of is involved in the construction of the potential. Effective twodimensional potentials with in the ground and first excited vibrational states are obtained by averaging a threedimensional potential for each case over the asymmetric stretch vibrational coordinate. Both potentials have only a Tshaped minimum with a well depth of 200.97 and , respectively. No linear local minima are detected. The radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm are employed to calculate the related rovibrational energy levels. The calculated band origin shift of the complex agrees very well with the observed one (−0.474 versus ). In addition, the predicted infrared spectra based on the two averaged potentials are in excellent agreement with the available experimental data, which again testifies the accuracy of the new potentials.

Theoretical spectroscopy of trans and isotopomers
View Description Hide DescriptionThe sixdimensional potential energy surface of the electronic ground state of trans is mapped at the RCCSD(T)/augccpV5Z level of theory. This potential energy surface is incorporated later into perturbative and variational treatments to solve the nuclear motion and to derive a set of spectroscopic data for trans, trans, and trans. Our vibrational spectra are compared with those deduced from the earlier photoelectron spectra by Frost et al. [J. Chem. Phys.64, 4719 (1976)], for which a good agreement between the theoretical and experimental results is found. Our calculations reveal the presence of strong anharmonic resonances between the vibrational levels of these cations even at low energies, thus complicating even more their assignment by vibrational quantum numbers. These resonances should participate in the transfer of intensities between the active modes during the direct photoionization of the neutral molecule and the combination modes and overtones of the inactive modes belonging to the totally symmetric irreducible representation.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Stability of supercooled binary liquid mixtures
View Description Hide DescriptionRecently, the supercooled Wahnström binary LennardJones mixture was partially crystallized into phase crystals in lengthy molecular dynamics simulations. We present molecular dynamics simulations of a modified Kob–Andersen binary LennardJones mixture that also crystallizes in lengthy simulations here, however, by forming pure fcc crystals of the majority component. The two findings motivate this paper that gives a general thermodynamic and kinetic treatment of the stability of supercooled binary mixtures, emphasizing the importance of negative mixing enthalpy whenever present. The theory is used to estimate the crystallization time in a Kob–Andersen mixture from the crystallization time in a series of related systems. At we estimate this time to be time units . A new binary LennardJones mixture is proposed that is not prone to crystallization and faster to simulate than the two standard binary LennardJones mixtures. This is obtained by removing the likeparticle attractions by switching to Weeks–Chandler–Andersen type potentials, while maintaining the unlikeparticle attraction.

OD stretch vibrational relaxation of HOD in liquid to supercritical
View Description Hide DescriptionThe population relaxation of the OD stretching vibration of HOD diluted in is studied by timeresolved infrared pumpprobe spectroscopy for temperatures between 278 and 663 K in the density range . Transient spectra recorded after exciting the OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant depends linearly on the static dielectric constant of water, indicating a correlation of with the average hydrogen bond connectivity of HOD within the network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in , the low density data of extrapolate to a nonzero intercept for . Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct transition, avoiding the excited HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel.