Volume 112, Issue 13, 01 April 2000
 COMMUNICATIONS


The detection of the free radical FO by submillimeterwave spectroscopy
View Description Hide DescriptionPure rotational transitions of the free radical FO in its ground electronic state have been detected by millimeterwave spectroscopy. Four transitions, which are all magnetic dipole in character, were measured in the frequency region from 219 to 408 GHz. The radical was produced by a wellknown efficient reaction between ozone and fluorine atoms. The high sensitivity available in the submillimeterwave region and a very efficient production method were essential for the detection of rotational transitions in this radical, which many other people have attempted in the past. No electric dipole transitions have been detected in this experiment. The precision of parameters associated with the rotational motion of FO has been improved considerably by fitting these lines together with previous data. Moreover, the paritydependent nuclear spin rotation term has been determined for the first time. The and structures of FO have been revised accordingly.

Preferential deuterium bonding at the ice surface: A probe of surface water molecule mobility
View Description Hide DescriptionInfrared spectra have been used to determine the temperature dependence of the preferential Dbonding of HDO at the singledonor (dangling O–H) sites on the ice surface. Data for icenanocrystals containing and HDO were in the form of the relative peak intensities of the O–D stretch mode bands of threecoordinated singledonor surface molecules of HDO and The magnitude of the enhanced stability of the Dbonded HDO molecules at these surface sites is estimated as This value matches, within experimental error, the literature value for the Dbonded dimer of HDO [Engdahl and Nelander, J. Chem. Phys. 86, 1819 (1987)]. The thermal equilibrium between the Hbonded and Dbonded configurations of HDO at these sites is lost upon cooling below 60 K. This identifies the temperature of onset of molecular rotation of the singledonor surface molecules as 60 K, on a time scale of The possibility exists for determination of the onset temperatures of more complex molecular motions on the surface of ice from related data. In that respect, the constant intensity of the O–D stretch mode band of singledonor molecules suggests that more complex surface motions, necessary for interchange with neighboring Hbonded molecules, are frozen at temperatures below 130 K.
 Top

 THEORETICAL METHODS AND ALGORITHMS


Energy decomposition analysis of intermolecular interactions using a blocklocalized wave function approach
View Description Hide DescriptionAn energy decomposition scheme based on the blocklocalized wave function (BLW) method is proposed. The key of this scheme is the definition and the full optimization of the diabatic state wave function, where the charge transfer among interacting molecules is deactivated. The present energy decomposition (ED), BLWED, method is similar to the Morokuma decomposition scheme in definition of the energy terms, but differs in implementation and the computational algorithm. In addition, in the BLWED approach, the basis set superposition error is fully taken into account. The application of this scheme to the water dimer and the lithium cation–water clusters reveals that there is minimal charge transfer effect in hydrogenbonded complexes. At the HF/augccPVTZ level, the electrostatic,polarization, and chargetransfer effects contribute 65%, 24%, and 11%, respectively, to the total bonding energy (−3.84 kcal/mol) in the water dimer. On the other hand, charge transfer effects are shown to be significant in Lewis acid–base complexes such as and In this work, the effect of basis sets used on the energy decomposition analysis is addressed and the results manifest that the present energy decomposition scheme is stable with a modest size of basis functions.

Temperature dependent reaction coordinates
View Description Hide DescriptionTemperaturedependent reactioncoordinates are investigated using Brownian dynamics. A functional of the reaction coordinate, which does not have explicit time dependence, is derived. The path that minimizes the functional is defined as the reaction coordinate. The optimal coordinate varies from the steepest descent path at zero temperature to a straight line connecting “reactants” and “products” at high temperatures. An estimate of the time scale of the process is an output of the optimization. A numerical example is provided and adjustments for the Stratonovich calculus are discussed.

Multireference perturbation theory for large restricted and selected active space reference wave functions
View Description Hide DescriptionA multireference secondorder perturbation theory (MRPT2) has been developed which allows the use of reference wave functions with large active spaces and arbitrary configuration selection. Internally contracted configurations are used as a basis for all configuration subspaces of the firstorder wave function for which the overlap matrix depends only on the secondorder density matrix of the reference function. Some other subspaces which would require the third or fourthorder density matrices are left uncontracted. This removes bottlenecks of the complete active space second order pertubation theory (CASPT2) caused by the need to construct and diagonalize large overlap matrices. Preliminary applications of the new method for 1,2dihydronaphthalene (DHN) and free base porphin are presented in which the effect of applying occupancy restrictions in the reference wave function (restricted active space secondorder perturbation theory, RASPT2) and reference configuration selection (general MRPT2) on electronic excitation energies is tested. In the case of the transition of DHN rapid convergence of the RASPT2 and MRPT2 excitation energies towards the CASPT2 value with increasing number of reference configurations is observed. In the calculations for the lowest five states of porphin all 24 valence πorbitals were included in the active space of restricted active space selfconsistent field (MCSCF) and MRPT2 calculations. From the RASSCF wave functions different subsets of reference configurations were selected on the basis of their coefficients. In this case convergence of the excitation energies with decreasing selection threshold is found to be rather slow, indicating the need for more elaborate selection schemes. The computed excitation energies are in good agreement with previous CASPT2 results obtained with much smaller active spaces.

Reaction field treatment of charge penetration
View Description Hide DescriptionTreatment of the important electrostatic effects of solvation by means of reactionfield theory is becoming common in electronic structure calculations on molecules. Most extant reaction field methods neglect or crudely approximate the often important influence of volume polarization arising from solute charge that quantum mechanically penetrates outside the cavity that nominally encloses it. This work proposes and examines a new formulation that provides an accurate simulation of volume polarization effects while being much simpler to implement and use than an exact treatment. Detailed comparisons with other related methods are also given.

Nonadiabatic photodissociation dynamics of ICN in the continuum: A semiclassical initial value representation study
View Description Hide DescriptionIn this paper we investigate the nonadiabaticphotodissociation dynamics of ICN in the continuum, using a semiclassical initial value representation method which is able to describe electronically nonadiabatic processes through the quantization of the classical electron–nuclear model Hamiltonian of Meyer and Miller [J. Chem. Phys. 70, 3214 (1979)]. We explore the capabilities of this semiclassical technique as applied to studying the ICNabsorptionspectrum, and the CN rotational distribution, through direct comparison of our semiclassical results with experimental data, and with full quantum mechanical calculations. We find that the Meyer–Miller Hamiltonian, quantized according to the semiclassical prescription, describes the ICNphotodissociation dynamics in excellent agreement with fullquantum mechanical calculations.

Magnetic coupling in neutral and charged and CrMn dimers
View Description Hide DescriptionTheoreticalab initio studies of neutral, cationic and anionic and CrMn dimers have been carried out to explore the progression of magnetic coupling with the number of electrons. It is shown that while and have antiferromagnetically coupled atomic spins, has a ferromagneticground state closely followed by an antiferromagnetic state. On the other hand, all dimers are ferromagnetic, irrespective of the charge. The neutral CrMn is ferrimagnetic while the charged CrMn are antiferromagnetic. In all cases, the charged dimers are found to be more stable than the neutral ones. The results are compared with available calculations and experiments and the difficulties associated with theoretical description and the experimental interpretations are discussed.

On apparent quantized transitionstate thresholds in the photofragmentation of acetaldehyde
View Description Hide DescriptionRecent photofragmentation experiments have observed stepwise increases in the dissociation rate for as a function of excitation energy. In accord with the Rice–Ramsperger–Kassel–Marcus (RRKM) form of transitionstate theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional(DFT) methods with [C,O/H] atomicorbital basis sets ranging in quality from to A highlevel focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, and the association barrier height, With a basis set of tripleζ plus doublepolarization plus quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of and respectively, while the empirical value inferred from RRKM models is only The RRKMderived frequencies for the degrees of freedom orthogonal to the reaction path are more reasonable but still not in convincing agreement with electronic structure theory. Thus, while the experimental steps in the dissociation rate of acetaldehyde (as well as ketene) have yet to be satisfactorily explained, proven ab initio methods provide strong evidence that simple RRKM fits to the profile provide misleading vibrational frequencies of the transition state on the corresponding triplet potential energy surface.

Xray Raman scattering under pulsed excitation
View Description Hide DescriptionAccounting for actual and anticipated developments in synchrotron radiation and in xray and free electron laser techniques, we present a theory of resonant scattering of shortpulsed x rays. Both the instantaneous and integratedoverobservationtime xray scattering spectra are studied. Contrary to the commonly accepted notion of an instantaneous probability, we find that the correct quantity describing the resonant xray scattering (RXS) experiment is the probability integrated over the time of measurement. It is shown that the integral RXS probability is given by the convolution of the RXS probability for monochromatic excitation with the Wiener–Khintchine spectrum of the nonstationary incident radiation. We have found that short pulses yield a new spectral band following a nonRaman dispersion law. The finite duration of pulses results in a broadening of the Raman resonance. Some aspects of the theoretical analysis of the RXS spectra as functions of the pulse duration time are analyzed by numerical calculations.

A CCSDT study of the effects of higher order correlation on spectroscopic constants. I. First row diatomic hydrides
View Description Hide DescriptionSpectroscopic constants have been determined for 13 first row diatomic hydrides using coupled clustertheory with explicit inclusion of (iterative) triple excitations (CCSDT). Comparison of the predicted dissociation energies, bond lengths, harmonic frequencies, and anharmonicities was made with experiment and other highlevel theoretical treatments. These include complete active space configuration interactionwave functions,coupled clustertheory with perturbative triples [CCSD(T)], and new benchmark full configuration interaction calculations. Excellent overall agreement with experiment was found, even without correcting for small changes due to core/valence and relativistic effects. The intrinsic CCSDT error with respect to experiment for each molecule and property was estimated by extrapolating to the complete basis set limit. Among the various properties examined in this study, no significant differences were found between CCSD(T) and CCSDT. In light of the substantial increase in computational cost associated with the latter method, there appears to be little justification for selecting it over CCSD(T) in studies of first row hydrides. Preliminary results for first row diatomics, e.g., suggest that the impact of CCSDT will increase with the number of electrons.

A study of the relative importance of one and twoelectron contributions to spin–orbit coupling
View Description Hide DescriptionThe existing methods to estimate the magnitude of spin–orbit coupling for arbitrary molecules and multiconfigurational wave functions are reviewed. The formfactor method is extended from the original singlet–triplet formulation into arbitrary multiplicities. A simplified version of the meanfield method (the partial twoelectron method, P2E) is formulated and tested versus the full twoelectron operator on a set of representative molecules. The change of the one and twoelectron spin–orbit coupling down the Periodic Table is investigated, and it is shown that the computationally much less demanding P2E method has an accuracy comparable to that of the full twoelectron method.

Convergence of Breit–Pauli spin–orbit matrix elements with basis set size and configuration interaction space: The halogen atoms F, Cl, and Br
View Description Hide DescriptionSystematic sequences of basis sets are used to calculate the spin–orbit splittings of the halogen atoms F, Cl, and Br in the framework of firstorder perturbation theory with the Breit–Pauli operator and internally contracted configuration interactionwave functions. The effects of both higher angular momentum functions and the presence of tight functions are studied. By systematically converging the oneparticle basis set, an unambiguous evaluation of the effects of correlating different numbers of electrons in the Cl treatment is carried out. Correlation of the electrons in chlorine increases the spin–orbit splitting by ∼80 cm^{−1}, while in bromine we observe incremental increases of 130, 145, and 93 cm^{−1}, when adding the and electrons to the set of explicitly correlated electrons, respectively. For fluorine and chlorine the final basis set limit, allelectrons correlated results match the experimentally observed spin–orbit splittings to within ∼5 cm^{−1}, while for bromine the Breit–Pauli operator underestimates the splitting by about 100 cm^{−1}. More extensive treatment of electron correlation results in only a slight lowering of the spin–orbit matrix elements. Thus, the discrepancy for bromine is proposed to arise from the nonrelativistic character of the underlying wave function.

A nonorthogonal approach to perfect pairing
View Description Hide DescriptionWe present an alternative formulation of perfect pairing (PP) aimed at giving a more faithful representation of the valence correlation energy of an arbitrary molecule. In the new theory, the occupied and virtual orbitals are nonorthogonal amongst themselves but orthogonal to each other. Whereas for the fully orthogonal version of PP one has the number of pairs equal to the number of occupied orbitals, the current formulation allows for an arbitrary number of pairs built from redundant orbitals. We propose setting the number of pairs equal to the number of valence orbitals in the molecule. Preliminary results indicate that the redundant formulation gives qualitatively improved results for delocalized systems such as benzene, while maintaining the attractive features of PP for localized systems.

An extensive study of gradient approximations to the exchangecorrelation and kinetic energy functionals
View Description Hide DescriptionWe formalize the procedure of functional development, in a general theoretical framework. Expansion in a functional basis set, and fitting via an error functional to a data set, casts functional development as a variational problem to obtain the functional basisset and dataset limits. Overfitting is avoided by defining the optimum number of parameters. We implement our theory for an investigation of first and secondorder generalized gradient approximations (GGA) to the exchangecorrelation and kinetic energy functionals, within an ab initio model. A variety of functional basis sets, including a general finiteelement representation, is constructed to represent both onedimensional and multidimensional GGA enhancement factors. An extensible data set consisting of 429 atomic and diatomic, neutral and cationic species, at stretched and equilibrium geometries, is constructed from Moller–Plesset level exchangecorrelation energies, and Hartree–Fock kinetic energies. The range of chemically relevant density and gradient variables is examined. Exhaustive fitting investigations are carried out, to determine the accuracy of the GGA representation of the ab initio models. In the exchangecorrelation case we demonstrate that we can reach the functional basisset and dataset limit, which correspond to a rootmeansquare (rms) error of mH (6.3 kcal/mol). Changing the functional basis set, higherorder density variables such as the kinetic energy density, multidimensional enhancement factors, and exact exchange yield no significant improvement, and our fits represent an effective solution of the GGA problem for exchangecorrelation, at the Møller–Plesset level. In the kinetic energy case, accurate functionals with rms errors of mH (50 kcal/mol) are developed. These exhibit a beautifully simple kinetic energy enhancement factor, and are a step towards orbitalfree calculations.
 Top

 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Nonresonant photofragmentation/ionization dynamics of using picosecond and femtosecond laser pulses at 248 nm
View Description Hide DescriptionPhotodissociation/ionization of molecular oxygen in a cold molecular beam is studied using a shortpulse laser beam at 248 nm and velocity map imaging. Both photoelectron and images are recorded for laser pulsewidths of 5 and 0.5 ps. Most of the observed ionizationdynamics takes place after absorption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postdissociative ionization creating atoms where is an electronically excited autoionizing atom, and molecular (auto)ionization to create a range of highly vibrationally excited ground electronic state ions. The observed signals then arise from resonanceenhanced twophotondissociation of or autoionization of atoms, while the electron signals arise from ionization of or autoionization of The latter channel can be used to directly scale the photoelectron and signal strengths. The images show strong differences for 0.5 ps and 5 ps pulse which could arise from ac Stark shifting of levels involved in the transition of With resonance enhancement, twophotondissociation of is the dominant process producing ions. The angular distributions show an anisotropy that is more extreme than a simple twostep dissociation, which is attributed to alignment effects.

Photodissociation of and below 132 nm
View Description Hide DescriptionA complete determination of the rotational and vibrational distributions in the fragments which result from the VUV photodissociation of as a function of excitation energy is presented. VUV excitation was performed at eight different wavelengths (eleven for in the range between 132 and 124 nm. The vibrational branching ratios show a relatively fast rise of the vibrational excitation at threshold followed by a plateau. The general trend of the experimental results is well reproduced by a phase space theory calculation and by ab initio calculations reported by van Hemert and van Harrevelt. Rotational distributions seem to depend only on the available excess energy and, for a given excess energy appear to be similar for all vibrational levels in OH and OD.

Quantum scattering study of collisional energy transfer in The importance of the vibronic mixing
View Description Hide DescriptionWe present the results of a quantum scattering study of collisional energy transfer in the title reaction, considering energies up to 14 000 cm^{−1} above the zero point energy. The collisions are described using the VCC–IOS (vibrationalcoupledchannel infiniteordersudden) quantum scattering method, with two coupled potential surfaces and as many as 329 total states in the basis expansion. The intramolecular potentials describe the and states of and their coupling through a conical intersection. The intermolecular potential is based on empirical sums of atom–atom potentials. We find that vibronic mixing between the X and A states of strongly enhances collisional energy transfer and, as a result, there is a noticeable change in the energy transfer efficiency as vibronic energy in is increased above the threshold for A state excitation. This change in efficiency occurs even if the same intermolecular potential is used for both electronic states. Many transitions are enhanced because the energy gaps between vibronically mixed and unmixed states are smaller. Energy transfer is also enhanced between states which are both vibronically mixed, whenever a significant component of each state involves the same zeroth order level of the excited electronic state. The calculated results are in good agreement with recent measurements.

Geometric phase effects in the reaction
View Description Hide DescriptionThe general vector potential (gauge theory) approach for including geometric phaseeffects in accurate threedimensional quantum scattering calculations in symmetrized hyperspherical coordinates is applied to the reaction at 126 values of total energy in the range 0.4–2.4 eV. Statetostate reaction probabilities, integral, and differential cross sections are computed using both the Boothroyd–Keogh–Martin–Peterson (BKMP2) and the Liu–Siegbahn–Truhlar–Horowitz potential energy surfaces for the first six values of total angular momentum Calculations are performed on each surface both with and without the geometric phase. Due to symmetry, the effects of the geometric phase are shown to cancel out when contributions from even and odd values of are added together for both the integral and differential cross sections, at allenergies, and independent of which surface is used. These results are consistent with recent experimental results which are in good agreement with theoretical treatments which do not include the geometric phase. Relatively broad transition state resonances are observed in the rotationally resolved integral and differential cross sections.

Electronhydrogen bonds and OH harmonic frequency shifts in water cluster complexes with a group 1 metal atom, (M=Li and Na)
View Description Hide DescriptionThe harmonic vibrational frequencies for (M=Li and Na) are calculated with ab initio molecular orbital (MO) methods. Three types of isomers, surface, quasivalence and semiinternal, exhibit the characteristic frequency shifts The calculated spectral patterns are related to the geometric conformations around the localized electron {e} in the cluster. The downward shifts of OH frequencies are strongly correlated with the lengthening of OH bond distances, as is well known in the hydrogen bonds. The magnitude of the downward shift is as large as that for the hydrogen bond. The correlation of the shifts with the distance between the center of the electron {e} and the hydrogen atom reveals two types of interaction schemes. The characteristics of the structural unit OH{e}HO in and its OH modes are common with those found in water anion clusters. The interaction between the localized electron and OH bonds can be called an electronhydrogen bond.
