Volume 130, Issue 15, 21 April 2009

2chloroethanol molecules having temperature from 300 to 600 K have been captured by helium droplets and the infrared spectra of the O–H and C–H stretching bands of the embedded molecules have been obtained. The intensity ratio of the bands due to trans and gauche conformers of the molecules follows the Arrhenius dependence, giving the enthalpy of conformers interconversion of 1.1 kcal/mol. It is concluded that the abundance ratio of the conformers, which are at equilibrium in the gas phase, remains unchanged upon the rapid cooling of the molecules in the helium droplets.
 ARTICLES

 Theoretical Methods and Algorithms

Quantum densities of states of fluxional polyatomic systems from a superposition approximation
View Description Hide DescriptionThe superposition method is used to calculate quantum densities of states of polyatomic systems with multiple isomeric structures. For each isomer, anharmonicities are included rigorously using a Dunham expansion of the vibrational energy levels and short exchange Monte Carlo simulations are used to compute the individual quantum densities of states. The method is applied to the computation of thermodynamicalproperties of the and clusters. The canonical heat capacities are found in very satisfactory agreement with the predictions of quantum or semiclassical sampling methods.

On the relation between timedependent and variational density functional theory approaches for the determination of excitation energies and transition moments.
View Description Hide DescriptionIt is shown that it is possible to derive the basic eigenvalue equation of adiabatic timedependent density functional theory within the Tamm–Dancoff approximation (TDDFT/TD) from a variational principle. The variational principle is applied to the regular Kohn–Sham formulation of DFTenergy expression for a single Slater determinant and leads to the same energy spectrum as TDDFT/TD. It is further shown that this variational approach affords the same electric and magnetic transition moments as TDDFT/TD. The variational scheme can also be applied without the Tamm–Dancoff approximation. Practical implementations of TDDFT are limited to second order response theory which introduces errors in transition energies for charge transfer and Rydberg excitations. It is indicated that higher order terms can be incorporated into the variational approach. It is also discussed how the current variational method is related to traditional DFT schemes based on variational principles such as , and how they can be combined.

General theory of excitation energy transfer in donormediatoracceptor systems
View Description Hide DescriptionGeneral theory of the excitation energy transfer (EET) in the case of donormediatoracceptor system was constructed by using generalized master equation (GME). In this theory, we consider the direct and indirect transitions in the EET consistently. Hence, our theory includes the quantum mechanical interference between the direct and indirect transitions automatically. Memory functions in the GME were expressed by the overlap integrals among the timedependent emission spectrum of the donor, the absorptionspectrum of the mediator, the timedependent emission spectrum of the mediator, and the absorptionspectrum of the acceptor. In the Markov limit of the memory functions, we obtained the rate of EET which consists of three terms due to the direct transition, the indirect transition, and the interference between them. We found that the interference works effectively in the limit of slow thermalization at the intermediate state. The formula of EET rate in this limit was expressed by the convolution of the EET interaction and optical spectra. The interference effect strongly depends on the width of the absorptionspectrum of mediator molecule and the energy gap between the donor and the mediator molecules.

An electronpreceding perspective on the deformation of materials
View Description Hide DescriptionElements of Bader’s theory of atoms in molecules are combined with densityfunctional theory to provide an electronpreceding perspective on the deformation of materials. From this perspective, a network of atoms is changed by moving the bonds that connect them; the nuclei then follow. The electronic stress tensor is the key to understanding this process. Eigenvectors of the electronic stress tensor at critical points of the electron density provide insight into the “normal electronic modes” that accompany structural dynamics and rearrangements. Eigenvectors of the secondderivative matrix of the electron density emerge as effective approximations to the eigenvectors of the stress tensor; this makes it possible to apply our results to experimentally and computationally determined electron densities. To demonstrate the usefulness of our analysis, we show that (a) the lowfrequency modes of ice Ic can be predicted from the eigenvectors of the secondderivative matrix and (b) the eigenvectors of the secondderivative matrix are associated with the direction of structural change during the pressureinduced phase transition from ice XI to a ferroelectricice VIIIlike structure. We conclude that the eigenvectors of the secondderivative matrix of the electron density are the key ingredient for constructing a dynamical theory of atoms in molecules.

Ab initio calculation of the electronic structures of the ground and and excited states of MnH
View Description Hide DescriptionElectronic structures and molecular constants of the ground and lowlying and electronic excited states of the MnH molecule were studied by multireference single and double excitation configuration interaction (MRSDCI) with Davidson’s correction calculations under exact symmetry using Slatertype basis sets. To correctly describe the electronic ground state,, at the calculation, we employed a large number of reference configurations in terms of the stateaveraged complete active space selfconsistent field (CASSCF) orbitals, taking into account the contribution from the excited state. The and states can well be described by the MRSDCI wave functions based on the CASSCF orbitals obtained for the lowest state only. In the , calculations of the , , and states required 16, 7, and 17 reference configurations, respectively. Molecular constants, i.e., and of these states and excitation energy from the state, obtained at the level, showed a good agreement with experimental values. The small remaining differences may be accounted for by taking relativistic effects into account.

Calculating electron paramagnetic resonance matrices for triplet state molecules from multireference spinorbit configuration interaction wave functions
View Description Hide DescriptionWe present a way to calculate electron paramagnetic resonance(EPR)matrices from variationally optimized spinorbit coupled wave functions. Our method constructs a triangular matrix from the matrix representation of the total electron magnetic moment in the basis of the spinorbit coupled wave functions by means of a projection technique. Principal values are obtained in the standard fashion by forming from the triangular matrix the tensor and diagonalizing it. In principle, the scheme allows to calculate the spinorbit orbital Zeeman cross term which usually gives the dominating contribution to the EPRshifts for any multiplicity. We have implemented this approach into a multireference spinorbit configuration interaction (MRSOCI) program [M. Kleinschmidt et al., J. Chem. Phys.124, 124101 (2006)]. Test applications are carried out for various triplet state sytems. The shifts of several of main group diatomics with ground state are investigated at the level of ab initio MRSOCI. We obtain perpendicular shifts which underestimate experimental values from literature by on the average. For a set of organic triplet state molecules we employ the combined density functional theory/multireference configuration interaction (DFT/MRCI) technique [S. Grimme and M. Waletzke, J. Chem. Phys.111, 5645 (1999)] to reduce the computational costs of the spinfree correlation problem. This approach yields principal values that match experiment well in many cases. Due to the small absolute shifts, a rigorous comparison will require the inclusion of firstorder contributions such as the relativistic mass correction and gauge correction terms which have not been included here. For the triplet state dication trans the principal shifts , , and are significantly larger and compare rather well to the experimental values , , and [A. Berces et al., Magn. Reson. Chem.37, 353 (1999)]. In comparison to conventional truncated sumover state techniques based on Rayleigh–Schrödinger perturbation theory, our new variational approach shows, in practice, robust and advantageous convergence characteristics with respect to the size of the manyparticle basis set. We demonstrate that the DFT/MRSOCI technology is a very feasible means to compute reliable shifts for large organic triplet systems at low computational cost.

Atomic Cholesky decompositions: A route to unbiased auxiliary basis sets for density fitting approximation with tunable accuracy and efficiency
View Description Hide DescriptionCholesky decomposition of the atomic twoelectron integral matrix has recently been proposed as a procedure for automated generation of auxiliary basis sets for the density fitting approximation [F. Aquilante et al., J. Chem. Phys.127, 114107 (2007)]. In order to increase computational performance while maintaining accuracy, we propose here to reduce the number of primitive Gaussian functions of the contracted auxiliary basis functions by means of a second Cholesky decomposition. Test calculations show that this procedure is most beneficial in conjunction with highly contracted atomic orbital basis sets such as atomic natural orbitals, and that the error resulting from the second decomposition is negligible. We also demonstrate theoretically as well as computationally that the locality of the fitting coefficients can be controlled by means of the decomposition threshold even with the longranged Coulomb metric. Cholesky decompositionbased auxiliary basis sets are thus ideally suited for local density fitting approximations.

Interfacial tension of nonassociating pure substances and binary mixtures by density functional theory combined with Peng–Robinson equation of state
View Description Hide DescriptionWe develop a density functional theory and investigate the interfacial tension of several pure substances , , , normal alkanes from to , and binary mixtures , , , , , , , , , , , and . The theory is combined with the semiempirical Peng–Robinson equation of state (PREOS). The weighted density approximation (WDA) is adopted to extend the bulk excess Helmholtz free energy to the inhomogeneous interface. Besides, a supplementary term, quadratic density expansion (QDE), is introduced to account for the longrange characteristic of intermolecular dispersion attractions, which cannot be accurately described by the WDA. In the bulk limit, the QDE vanishes and the theory is reduced to the PREOS. For pure substances, the potential expansion parameter is the only adjustable parameter in the QDE and determined by using a single measured interfacial tension at the lowest temperature examined. Then without any parameter adjustment, we faithfully predict the interfacial tensions of pure substances and mixtures over a wide range of conditions.

Quasilinearity in tetratomic molecules: An ab initio study of the CHNO family
View Description Hide DescriptionWe present coupledcluster CCSD(T) all electron results for the equilibrium structure of isofulminic acid, HONC, together with results for the barrier to linearity and the energetics for the four most stable members of the CHNO isomer family, obtained for the ground electronic states by means of large correlation consistent basis sets. Minimum energy paths along the angular coordinates reported for these CHNO isomers are combined with the dominant kinetic energy contributions to predict key rovibrational spectroscopic features which are clearly reminiscent of quasilinear behavior in tetratomic molecules.

A onedimensional dipole lattice model for water in narrow nanopores
View Description Hide DescriptionWe present a recently developed onedimensional dipole lattice model that accurately captures the key properties of water in narrow nanopores. For this model, we derive three equivalent representations of the Hamiltonian that together yield a transparent physical picture of the energetics of the water chain and permit efficient computer simulations. In the charge representation, the Hamiltonian consists of nearestneighbor interactions and Coulomblike interactions of effective charges at the ends of dipole ordered segments. Approximations based on the charge picture shed light on the influence of the Coulomblike interactions on the structure of nanopore water. We use Monte Carlo simulations to study the system behavior of the full Hamiltonian and its approximations as a function of chemical potential and system size and investigate the bimodal character of the density distribution occurring at small system sizes.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

A theoretical investigation on the spectrum of the Ar trimer for high rotational excitations
View Description Hide DescriptionA detailed study of the rovibrational spectrum of the Ar trimer is performed by means of an exact hyperspherical coordinate (HC) method and a variational approach based on distributed Gaussian functions (DGFs) to describe the interparticle distances. The good agreement observed between the energy levels obtained with both procedures for high values of the total angular momentum ( and 20) reveals the quality of the DGF method to describe the rotation of the title system. Rotational constants for the lowest bound states, obtained as averages for each vibrational state, have been obtained and compared to previous results. A detailed analysis of density probability functions obtained by means of the HC approach for rovibrational states at and 20 shows close similitudes thus supporting the vibrationrotation separation adopted within the DGF scheme for the system.

Reversible structure transformation in ice nanocluster
View Description Hide DescriptionWater freezes to ice by cooling. Once ice is formed, the phase does not change unless heated over the melting point. This is a familiar phenomenon observed in our everyday life. Here, we demonstrate nonbulklike behavior of nanosized ice by moleculardynamics simulation using the TIP4P potential. Our simulations reveal that the structure of the nanocluster reversibly changes and a variety of solidlike phases, such as bilayer structures composed of three pentagonal prisms, square and pentagonal icenanotubes, and stuffedfullerenelike structures, dynamically coexists even at 52% of the bulk melting point.

The waterbenzene interaction: Insight from electronic structure theories
View Description Hide DescriptionWeak noncovalent interactions such as van der Waals and hydrogen bonding are ubiquitous in nature, yet their accurate description with electronic structuretheories is challenging. Here we assess the ability of a variety of theories to describe a waterbenzene binding energy curve. Specifically, we test Hartree–Fock, secondorder Møller–Plesset perturbation theory, coupled cluster,density functional theory with several exchangecorrelation functionals with and without empirical vdW corrections, and quantum Monte Carlo (QMC). Given the relative paucity of QMC reports for noncovalent interactions, it is interesting to see that QMC and coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] are in very good agreement for most of the binding energy curve, although at short distances there are small deviations on the order of 20 meV.

Structures of water and methanol cluster ions: and
View Description Hide DescriptionInfrared photodissociation (IRPD) spectra of and are measured in the region. At the same time, the solvation characteristics in the clusters are investigated theoretically; the geometry optimization and the vibrational analysis are carried out for the and the ions at the level of theory. The IRPD spectrum of the ion shows the free OH and the hydrogenbonded OH stretching bands of the ion core and the antisymmetric CO stretching band of the solvent molecule, indicating that the solvent molecule is preferentially solvated to the ion core via the hydrogen bond. In , the free OH stretching band is not observed; both of the OH groups of the ion core are hydrogen bonded to the solvent molecules. Spectral features of the IRPD spectra of suggest that the third and the fourth molecules are bound to the oxygen atom of the ion core, and that the first solvation shell of the ion core becomes filled with four molecules. All the IRPD spectra of the ions display the hydrogenbonded OH stretching band of the ion core, meaning that the solvent molecule is preferentially bonded to the OH group of the ion core, similar to the case of . Quantum chemical calculations for the ions demonstrate that the second and the third solvent molecules are bonded to the oxygen atom of the ion core.

Ultrafast electron dynamics following outervalence ionization: The impact of lowlying relaxation satellite states
View Description Hide DescriptionLowlying relaxation satellites give rise to ultrafast electron dynamics following outervalence ionization of a molecular system. To demonstrate the impact of such satellites, the evolution of the electronic cloud after sudden removal of an electron from the highest occupied molecular orbital (HOMO) of the organic unsaturated nitroso compound 2nitroso[1,3]oxazolooxazole is traced in real time and space using ab initio methods only. Our results show that the initially created hole charge remains stationary but on top of it the system reacts by an ultrafast excitation followed by a cyclic excitationdeexcitation process which leads to a redistribution of the charge. The excitation following the removal of the HOMO electron takes place on a subfemtosecond time scale and the period of the excitationdeexcitation alternations is about 1.4 fs. In real space the processes of excitation and deexcitation represent ultrafast delocalization and localization of the charge. The results are analyzed by simple two and threestate models.

Synchrotron photoionization mass spectrometry study of intermediates in fuelrich 1,2dimethoxyethane flame
View Description Hide DescriptionIntermediates in a fuelrich premixed laminar 1,2dimethoxyethane (DME)flame are studied by molecular beammass spectrometry combined with tunable synchrotron vacuum ultraviolet photoionization. About 30 intermediate species are identified in the present work, and their mole fraction profiles are evaluated. The experimental results show that the formations of intermediates, both hydrocarbons and oxygenated hydrocarbons, are closely linked to the structure of fuel, which is consistent with the previous reports. Species produced from H atom abstraction and beta scission of DME usually have much higher concentrations than others. The oxygen atoms in DME are considered to act as partitions of the primary intermediates; therefore farther reactions among these primary intermediates are difficult to occur, resulting in absence of most large intermediate species.

Nuclear dynamics during the resonant Auger decay of water molecules
View Description Hide DescriptionThe resonant Auger decay of water molecules is investigated. Here, the excitation process, the motion of the nuclei, and the decay of the resonantly excited state take place on the same (femtosecond) time scale. Therefore, a multistep picture is not suitable. Instead, the nuclear wave packet at each instant of time is a result of several competing and interfering contributions. The resonant Auger decay of water is simulated and its dynamics is studied in detail. An analysis of the final vibrational distribution is given. The multiconfiguration timedependent Hartree method is used to study the intricate multidimensional dynamics. The potential energy surfaces have been calculated using a multireference configuration interaction method.

The electronic structure and bonding of AlNAl
View Description Hide DescriptionWe have studied 16 states, 7 doublets, and 9 quartets of the AlNAl isomer mainly through multireference methods and correlation consistent basis sets. We report equilibrium geometries, energetics, and dipole moments, whereas for a number of low lying states we have constructed dissociation AlN–Al potential energy profiles. For the same states we also analyze their bonding character using valence–bond–Lewis diagrams. Our results are consistent with the limited experimental data.

Valencebond description of chemical reactions on Born–Oppenheimer molecular dynamics trajectories
View Description Hide DescriptionThe nature of chemical bonds on dynamic paths was investigated using the complete active space valencebond (CASVB) method and the Born–Oppenheimer dynamics. To extract the chemical bond picture during reactions, a scheme to collect contributions from several VB (resonance) structures into a small numbers of indices was introduced. In this scheme, a tree diagram for the VB structures is constructed with the numbers of the ionic bonds treated as generation. A pair of VB structures is related to each other if one VB structure is transferred into the other by changing a covalent bond to an ionic bond. The former and latter VB structures are named parent and child structures, respectively. The weights of the bond pictures are computed as the sum of the CASVB occupation numbers running from the top generation to the bottom along the descent of the VB structures. Thus, a number of CASVB occupation numbers are collected into a small number of indices, and a clear bond picture may be obtained from the CASVB wave function. The scheme was applied to the hydrogen exchange reaction and the Diels–Alder reaction. In both the reactions, the scheme gave a clear picture for the Born–Oppenheimer dynamics trajectories. The reconstruction of the bonds during reactions was well described by following the temporal changes in weight.

Fieldfree molecular alignment for studies using xray pulses from a synchrotron radiation source
View Description Hide DescriptionA short, intense laser pulse may be employed to create a spatially aligned molecular sample that persists after the laser pulse is over. We theoretically investigate whether this impulsive molecular alignment technique may be exploited for experiments using xray pulses from a thirdgeneration synchrotron radiation facility. Using a linear rigid rotor model, the alignment dynamics of model molecular systems with systematically increasing size is calculated utilizing both a quantum density matrix formalism and a classical ensemble method. For each system, the alignment dynamics obtained for a 95 ps laser is compared with that obtained for a 10 ps laser pulse. The average degree of alignment after the laser pulse, as calculated quantum mechanically, increases with the size of the molecule. This effect is quantitatively reproduced by the classical calculations. The average degree of impulsive alignment is high enough to induce a pronounced linear dichroism in resonant xrayabsorption using the intense 100 ps xray pulses currently available. However, for structural studies based on elastic xray scattering, bright xray pulses with a duration of 1 ps or shorter will be required in order to make full use of impulsive molecular alignment.