Volume 113, Issue 19, 15 November 2000
 COMMUNICATIONS


Direct ab initio variational calculation of vibrational energies of the complex and resolution of experimental differences
View Description Hide DescriptionWe report the direct ab initio calculation of vibrational energies of the chloride anion–water complex by interfacing the code MULTIMODE, which does variational calculations of vibrational energies, with GAUSSIAN, which does ab initio calculations of electronic energies. Convergence of the results with respect to the level of modecoupling considered indicates that the present results are reliable enough to distinguish between two sets of conflicting experimental reports of these vibrational energies.
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 FOCUS ARTICLES


Triplet state solvation dynamics: Basics and applications
View Description Hide DescriptionApplying solvation dynamics experiments to viscous liquids or glassy materials near their glass transition involves long lived triplet probes, whose time dependent phosphorescence signals depend upon the local dipolar orientational dynamics, mechanical responses, and polarities. The current understanding of experimental results regarding steady state and time dependent optical line shapes and positions is reviewed with emphasis on the relation to the macroscopic dielectric properties. Several applications are discussed in detail, where advantage is taken of the spatially local instead of ensemble averaging character of this technique. These examples include studies of dynamical heterogeneity, rotational solute/solvent coupling, secondary relaxations in the glassy state, as well as confinement and interfacial effects.
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 ARTICLES

 Theoretical Methods and Algorithms

Relative energies of the isomers 1,2dithiete and dithioglyoxal: Peculiar basis set dependencies of density functional theory and ab initio methods
View Description Hide DescriptionAb initio calculations at the levels of Hartree–Fock (HF), second order Møller–Plesset perturbation theory (MP2), coupledcluster theory with singles, doubles, and estimated triple excitations [CCSD(T)], and density functional theory(DFT) using the functionals B3LYP and B3PW91 of the relative energies of the isomers 1,2dithiete (2a), and dithioglyoxal (2b) show a peculiar dependence of the results on the ftype polarization functions. The ab initio calculations with basis sets with incorrectly predict that 2a is higher in energy than 2b. The relative energies at the MP2 and CCSD(T) levels change by more than 6 kcal/mol in favor of 2a if the basis set is augmented by one set of f functions. The DFT calculations also give a higher stability of 2a relative to 2b if f functions are included in the basis sets, but the change in the relative energy is only ∼2 kcal/mol. The large change in the relative energies which are calculated at MP2 and CCSD(T) are mainly due to the functions at sulfur, while the effect of the f functions in the DFT calculations is mainly due to the f functions at carbon.

The ground state of harmonium
View Description Hide DescriptionA detailed analysis that benefits from a slate of new approximate numerical and exact asymptotic results produces highly accurate properties of the ground state of the harmonium atom as functions of the confinement strength ω and quantifies the domains of the weakly and strongly correlated regimes in this system. The former regime, which encompasses the values of ω greater than is characterized by the oneelectron density with a global maximum at In contrast, the harmonium atom within the latter regime, which corresponds to differs fundamentally from both its weakly correlated counterpart and Coulombic systems. Resembling a Wigner crystal of a homogeneous electron gas, it possesses a radially localized pair of angularly correlated electrons that gives rise to with a “fat attractor” composed of a cage critical point and a (1, −1) critical sphere. Allowing for a continuous variation in ω, the new compact representation of the groundstatewave function and accurate approximants for the corresponding electronic properties are designed to facilitate the use of harmonium in research on electron correlation and density functionals.

Simulation and extrapolation of coexistence properties with singlephase and twophase ensembles
View Description Hide DescriptionA unified framework is developed to describe extrapolations from simulations performed in either a single system box or two subsystems at equilibrium (i.e., Gibbs ensembles). It is shown that the Gibbs ensemble can be used in conjunction with histogram reweighting and pseudoensemble techniques in order to map out more effectively different kinds of phase diagrams, in particular for binary and ternary systems. These extrapolation schemes allow the use of different phaseequilibrium specifications, some of which could not be simulated by conventional approaches. Novel semiopen and osmotic Gibbs ensembles are also described as counterparts of singlephase open and osmotic ensembles, respectively. Applications of the proposed methods are presented to the simulation of pressurecomposition diagrams, bubble lines, and isoenthalpic partitioning.

On the systematic improvement of fixednode diffusion quantum Monte Carlo energies using pair natural orbital CI guide functions
View Description Hide DescriptionWhile the diffusionquantum Monte Carlo method (DQMC) is capable, in principle, of calculating exact ground state energies, in practice the fixednode (FN) approximation leads to node location errors which make FNDQMC energies upper bounds. It is shown that the node location error can be reduced systematically and without prohibitive increase of computer time requirements by using nodes derived from pair natural orbital CI wave functions (PNOCI). The reduction is demonstrated for the N atom and the molecules and With the DQMC/PNOCI method, we obtain a variational energy of −109.520(3) H for the molecule and −76.429(1) H for the ground state of the water molecule which is only 22 and 9 mH above the estimated nonrelativistic ground state energy, respectively.

Analytical unitary bounds on quantum dynamics: Design of optimum NMR experiments in twospin systems
View Description Hide DescriptionWe report analytical solutions to the unitary bound problem for coherence/polarization transfer in twospin systems by means of unitary operations. Theoretical upper bounds for the transfer efficiency along with the associated optimum transformation operators are obtained analytically by decomposing the unitary operator as a product of exponentials in the special unitary Lie group Addressing NMR spectroscopy as a specific example, the method is demonstrated for the nonHermitian transfers and being relevant for heteronuclear singlequantum coherence (HSQC) experiments as well as the double to singlequantum transfer being representative for coherenceorder and spinstateselective transfer in INADEQUATE CRexperiments. Furthermore, using a Lagrangian function approach it is demonstrated how the method enables analytical description of twodimensional bounds for cross polarization.

A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field
View Description Hide DescriptionA systematic method to fit calculated to observed vibrational frequencies has been developed and implemented in a computer program. The procedure consists of the refinement of a scaled quantum mechanical force field (SQMFF) previously obtained according to Pulay’s method. The key step in the process is the generation of an intermediate matrix, which is then refined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. This scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force fields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests.

Excitation energies of dissociating A problematic case for the adiabatic approximation of timedependent density functional theory
View Description Hide DescriptionTimedependent density functional theory (TDDFT) is applied for calculation of the excitation energies of the dissociating molecule. The standard TDDFT method of adiabatic local density approximation (ALDA) totally fails to reproduce the potential curve for the lowest excited singlet state of Analysis of the eigenvalue problem for the excitation energies as well as direct derivation of the exchangecorrelation (xc) kernel shows that ALDA fails due to breakdown of its simple spatially local approximation for the kernel. The analysis indicates a complex structure of the function which is revealed in a different behavior of the various matrix elements (between the highest occupied Kohn–Sham molecular orbital and virtual MOs as a function of the bond distance The effect of nonlocality of is modeled by using different expressions for the corresponding matrix elements of different orbitals. Asymptotically corrected ALDA (ALDAAC) expressions for the matrix elements are proposed, while for other matrix elements the standard ALDA expressions are retained. This approach provides substantial improvement over the standard ALDA. In particular, the ALDAAC curve for the lowest singlet excitation qualitatively reproduces the shape of the exact curve. It displays a minimum and approaches a relatively large positive energy at large ALDAAC also produces a substantial improvement for the calculated lowest triplet excitation, which is known to suffer from the triplet instability problem of the restricted KS ground state. Failure of the ALDA for the excitation energies is related to the failure of the local density as well as generalized gradient approximations to reproduce correctly the polarizability of dissociating The expression for the response function χ is derived to show the origin of the fieldcounteracting term in the xc potential, which is lacking in the local density and generalized gradient approximations and which is required to obtain a correct polarizability.

The activespace equationofmotion coupledcluster methods for excited electronic states: The EOMCCSDt approach
View Description Hide DescriptionThe idea of selecting the most important higherthandoubly excited configurations in singlereference coupledcluster (CC) calculations for quasidegenerate ground states of molecular systems through the use of active orbitals is extended to excited electronic states via the equationofmotion (EOM) CC formalism. The resulting EOMCCSDt method, in which triexcited clusters and the corresponding threebody components of the EOMCC excitation operator R are restricted to internal and semiinternal components defined through active orbitals, is capable of significantly improving the vertical excitation energies obtained with the conventional EOMCCSD (EOMCC singles and doubles) approach at a fraction of the computer cost associated with the full EOMCCSDT (EOMCC singles, doubles, and triples) calculations. The results of pilot calculations for the and molecules indicate that the EOMCCSDt method using small active spaces is as accurate as the EOMCCSDT approach. In particular, the EOMCCSDt method is capable of accurately describing states that are doubly excited relative to the reference state.

New scheme for solving the amplitude equations in the statespecific coupled cluster theory with complete active space reference for ground and excited states
View Description Hide DescriptionA new scheme for solving the coupled cluster (CC) amplitude equations for ground and excited electronic states in the stateselective multireference (MR) CC method based on the complete active space (CAS) reference wave function (CASCC) is proposed. The CASCC wave function is generated using a single formal reference determinant, which is one of the CAS determinants, as the origin of the configuration expansion. Some single and double excitations from other CAS determinants may be triple, quadruple, and higher excitations from the reference determinant. We show that one may include the contribution from these higher excitations indirectly by modifying the coupled cluster amplitude equations corresponding to the single and double excitations. The modification involves including projections against the higher excitations in the equations for the singles and doubles. Test calculations for the ground and the first excited state of the model system and for the singlet–triplet splitting of the biradical shows that the results improve with the use of the new scheme and become to the FCI benchmarks.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Accurate atomic line wavelengths from astronomical sky spectra
View Description Hide DescriptionNumerous atomic lines appearing in the terrestrial nightglow can be measured by HIRES, the echelle spectrometer on the 10 meter Keck I telescope on Mauna Kea. The observable nightglow lines include emissions from Na, K, Hg, Ne, N, O, and H. Agreement between the line positions and those from National Institute of Science and Technology (NIST) compilations is excellent for wellknown lines, typically 2 mÅ or better. For lines which are not wellknown, or cannot be measured directly in the laboratory, deviations are significant. In particular, for the optically forbidden transition the differences are substantial, 20 and 24 mÅ for the two components. Apart from improving the line positions for this transition, we also correct an error that has been perpetuated in the aeronomic literature for the last 30 years concerning the transition wavelengths. The potassium line, recently discovered in the HIRES sky spectra, exhibits a position difference of 11 mÅ between the NIST and HIRES values, significant for astronomical applications. The HIRES value is shown to be in close agreement with the latest laboratory study. For the Balmer series of H lines, we report the first groundbased observation of Hγ in the geocorona, and we confirm that the positions of the lines agree with the expectation that the principal source is direct solar resonance excitation, with a 5%–6% cascading contribution in the case of Hα. The absolute average intensities of the three lines are 3600, 900, and ∼210 millirayleighs (mR), respectively.

Hydration shell structure of the clusters from a model potential energy function
View Description Hide DescriptionThe structural properties of the hydrated hydroxide ion are studied in terms of a manybody potential energy function that has been parameterized according to the experimentally determined [Arshadi et al., J. Phys. Chem. 74, 1475, 1483 (1970)] enthalpy and entropy changes for the first five association reactions of the ion with Clusters in the size range are examined through a canonical Monte Carlo simulation at The resultant structures, irrespective of the cluster size, are predominantly linear of the dendrite type, with the first shell consisting of two water molecules. Minimum energy structures at for and 3 compare well with ab initioconformations.

A theoretical and experimental study of the ethylpaminobenzoate complexes
View Description Hide DescriptionWeakly bound hydrogen bonded ethylpaminobenzoate/water complexes, referred to henceforth both as or by their stoichiometry have been investigated with a combined approach of mass and light detector laser spectroscopic techniques and ab initio calculations. The experimental studies follow explorations with laser induced fluorescence(LIF), and include onecolor resonant enhanced multiphoton ionization (REMPI), twocolor REMPI (R2PI), pressure dependent R2PI and hole burning (HB) spectroscopies. Calculations were conducted at the level and for the 1:1 complex led to the existence of six stable isomers, identified as the experimental origin bands at +4, +6, +13, +89, +96, and +108 cm^{−1} above the bare EAB transition. It has been shown that three of these bands originate in the EAB trans conformer, while the other three derive from the EAB gauche conformer. None of the experimental methods used lead us to observe the complex spectrum and the inspection of the REMPI and R2PI spectra has been shown to be a fragmentation from the complex. The structures and identification of the set of isomers are reported and a comparison with the results on the family complexes methylpaminobenzoate/water is discussed.

Experimental and theoretical study of methylpaminobenzoate/ammonia complexes. I.
View Description Hide DescriptionMethylp complex, henceforth prepared in a pulsed supersonic expansion, has been examined by laser massselective spectroscopies and density functional theory calculations, aiming to ascertain its isomer number, structures, identification, ionization energies, and vibrational assignments. Resonance enhanced multiphoton ionization and hole burning spectra of the species in supersonic beams show two transitions redshifted by −715 and −709 cm^{−1} from that of bare MAB band origin and are plausibly associated with two different isomers, whereas ab initio calculations indicate the likely existence of five stable isomer structures. Identification of the experimental isomer spectra with the calculated structures is reported and, in particular, several isomer vibrational bands are identified by contrast with the calculated modes. Properties and features of the are compared with those of the MAB/water complexes.

Experimental and theoretical study of methylpaminobenzoate/ammonia complexes. II.
View Description Hide DescriptionA complementary laser spectroscopy and computational study of the complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm^{−1} from the bare MAB transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced twocolor photo ionization (R2PI) studies. “Hole burning” spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the 1:4 with a four ammonia chain coordinated to the group. The good agreement between calculated and experimental vibrational frequencies confirms the ammonia ring structure and allows us to assign a number of inter and intramolecular vibrational bands.

A complete active space selfconsistent field multireference configuration interaction study of the lowlying excited states of BrO
View Description Hide DescriptionA theoretical study of the excited states of the BrO radical has been carried out for the first time using high level ab initio molecular orbital methods. The vertical excitation energies for the lowlying excited states and are calculated using the internally contracted multireference configuration interaction (MRCI) method. Based on vertical excitation energies, the ordering of the first few excited states are determined to be and The potential energy curves for the ground state and several lowlying excited states are examined at the level of theory. Several excited states are calculated to intersect the state, resulting in predissociation into Results for the quartet states of BrO are also included, as well as calculations of the effects of spin–orbit coupling on the electronic states of BrO.

Collisioninduced dissociation dynamics in collisions: The role of electronic excitation
View Description Hide DescriptionIntegral collisioninduced dissociation (CID) cross sections at centerofmass (CM) energies ranging from threshold to 22 eV and product ion recoil velocity distributions are presented for collisions. The ions are produced in an electron impact ion source and thus have ∼0.5 eV of vibrational energy. The CID cross sections are observed at energies below the 0 K thermodynamic threshold of 6.66 eV, implying the possibility of significant vibrational effects. A second CID threshold is apparent at This is close to the chargetransfer (CT) threshold energy. No CT products are observed. The CID cross section rises rapidly above a relative translational energy onset of and reaches a maximum value of ∼0.8 Å^{2} at 14 eV (CM). A CT onset of is observed which is close to the thermodynamic threshold for CID. The energy dependence of the CT cross section is very similar to the CID cross section except for a small step at energies where CID becomes efficient. The CID recoil velocity distributions near threshold are centered at the CM velocity and shift to forward preference at higher energies. The CT recoil velocities are forward scattered near threshold and become centered with respect to the CM velocity at energies where competition with CID is apparent.

Vibrational relaxation in pyridine upon supersonic expansion
View Description Hide DescriptionThe rotational spectra of five vibrational states of pyridine have been assigned and measured by millimeter waveabsorption spectroscopy in a supersonic expansion. The intensities of the lines of the vibrational satellites with respect to the ground state after the supersonic expansion depend on the kind of carrier gas, backing pressure, pyridine concentration, and symmetry of the rotational and vibrational states. Several rotational transitions of the vibrational satellites have also been measured in a conventional cell to complete the spectral assignment.

Reactions of with OHisoprene adduct isomers: Exothermicity, product ions, and rate constants
View Description Hide DescriptionThe ionmolecule reaction has been investigated in order to develop detection scheme for the OHisoprene adduct radicals using the chemical ionizationmass spectrometry (CIMS) method. Ab initio molecular orbital calculations have been employed to determine the structures and energies of the product ions from the charge transfer reactions of with the OHisoprene adduct radicals. Geometry optimizations of the OHisoprene adduct cations were performed with density function theory (DFT)B3LYP calculations and the energies were computed with the singlepoint calculation using the coupledcluster theory with single and double excitations including perturbative corrections for the triple excitations [CCSD(T)]. At the CCSD(T)/6311G^{*} ^{*} level of theory with corrected zeropointenergy (ZPE), the ionization potentials of the four OHisoprene adduct isomers with OH additions to C1 to C4positions (isomers I to IV) are 6.9, 5.6, 6.0, and 7.1 eV, respectively. The ionmolecule collision rate constants between and the OHisoprene adduct radicals were estimated using the averagedipoleorientation (ADO) theory, along with our calculated the permanent dipole moment and polarizability for the OHisoprene adduct isomers. We also report direct measurements of the ionmolecule reactionrate constant using the CIMS method, which are in good agreement with the predicted values using the ADO theory.