Volume 115, Issue 3, 15 July 2001
 COMMUNICATIONS


Exploration of conformational phase space in polymer melts: A comparison of parallel tempering and conventional molecular dynamics simulations
View Description Hide DescriptionParallel tempering molecular dynamics simulations have been performed for 1,4polybutadiene polymer melts in the 323 K–473 K temperature domain at atmospheric pressure. The parallel tempering approach provides a vast improvement in the equilibration and sampling of conformational phase space for the atomistic melt chains in comparison with conventional molecular dynamics simulations even for molecular weights and temperatures considered to be routinely accessible via the latter technique.
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 ARTICLES

 Theoretical Methods and Algorithms

Comparison of four methods to compute the dielectric permittivity of liquids from molecular dynamics simulations
View Description Hide DescriptionFour methods to compute the dielectricpermittivity ε of a liquid from molecular simulations are compared in the context of the simple point charge (SPC) water model. In the first method (unrestrained method), ε is evaluated from the fluctuations of the box dipole momentM, monitored during a single equilibrium simulation. In the three other methods, ε is evaluated from the probability distribution of the dipole moment norm. This distribution is itself evaluated in three different ways: (i) from multiple simulations involving a Mdependent biasing potential (umbrellasampling method), (ii) from multiple simulations involving a constrained dipole moment norm (Mconstraint method), or (iii) from fitting of incomplete estimates to a Maxwell distribution (fitting method). The four methods are shown to converge to an identical estimate of ε=61±1 for SPC water (256 molecules, reactionfield electrostatics). The convergence properties, advantages, and drawbacks of the different methods are analyzed in detail.

Convergence behavior of the symmetryadapted perturbation theory for states submerged in Pauli forbidden continuum
View Description Hide DescriptionThe polarization expansion and the symmetryadapted perturbation theory (SAPT) in the symmetrized Rayleigh–Schrödinger (SRS) and the Hirschfelder–Silbey (HS) formulations are applied through high order to the medium and longrange interaction of the groundstatelithium and hydrogen atoms. The interaction energies obtained by perturbation theory are compared with the counterpoisecorrected full configuration interaction results. It is shown that the SRS and HS expansions diverge as a result of the presence of the Pauli forbidden continuum in which the physical eigenstates of the perturbed Hamiltonian are submerged. Despite this divergence, the SAPT expansions give accurate results in low orders and excellent results when summed up in a standard way of assigning a sum to an asymptotically convergent series. The polarization expansion is found to diverge as well, with its asymptotic limit equal to the arithmetic mean of singlet and triplet energies. Unlike the case of simpler systems, for the interaction of lithium and hydrogen atoms the Hirschfelder–Silbey method does not provide any improvement over the much simpler SRS approach.

One and twocenter energy components in the atoms in molecules theory
View Description Hide DescriptionThe energy decomposition scheme proposed in a recent paper has been realized by performing numerical integrations. The sample calculations carried out for some simple molecules show excellent agreement with the chemical picture of molecules, indicating that such an energy decomposition analysis can be useful from the point of view of connecting quantum mechanics with the genuine chemical concepts.

Quantum dynamics using a discretized coherent state representation: An adaptive phase space method
View Description Hide DescriptionWe introduce a discretized coherent state representation (DCSR) for quantum dynamics. Expansion of a wave function in the nonorthogonal slightly overcomplete set is made with an identity operator computed using an iterative refinement method. Calculating the inverse of the overlap matrix is not necessary. The result is an accurate and efficient representation, where you only put basis functions in the region of phase space where the wave function is nonvanishing. Compared to traditional spatial grid methods, fewer grid points are needed. The DCSR can be viewed as an application of the Weyl–Heisenberg frame and extends it into a useful computational method. A scheme for fully quantum mechanical propagation is constructed and applied to the realistic problem of highly excited vibration in the heavy diatomic molecule Compared to splitoperator propagation in a conventional spatial grid, an order of magnitude longer time steps can be taken and fewer grid points are needed. The computational effort scales linearly with the number of basis functions. Nonreflecting boundary conditions are a natural property of the representation and is illustrated in a model of predissociation.

Atomic properties from energyoptimized wave functions
View Description Hide DescriptionMost of the variational Monte Carlo applications on quantum chemistry problems rely on varianceoptimized wave functions. Recently, M. Snajdr and S. M. Rothstein, [J. Chem. Phys. 112, 4935 (2000)] have concluded that energy optimization allows one to obtain wave functions that provide better values for a wide variety of ground stateproperties. In this work we study the quality of energyoptimized wave functions obtained by using the methodology of Lin, Zhang, and Rappe [J. Chem. Phys. 112, 2650 (2000)], as compared with varianceoptimized ones for He to Ne atoms. In order to assess this problem we calculate the energy and some other selected properties. The accuracy and performance of the energyoptimization method is studied. A comparison of properties calculated with energyoptimized wave functions to those existing in the literature and obtained by means of varianceoptimized wave functions shows a better performance of the former.

Comparison of full multiple spawning, trajectory surface hopping, and converged quantum mechanics for electronically nonadiabatic dynamics
View Description Hide DescriptionWe present calculations employing the simplest version of the full multiple spawning method, FMSM or minimal FMS, for electronically nonadiabatic quantum dynamics using three modelpotential energy matrices with different strengths and ranges for the diabatic coupling. We first demonstrate stability of the branching probabilities and final energy distributions with respect to the parameters in the FMSM method. We then compare the method to a variety of other semiclassical methods, as well as to accurate quantum mechanical results for threedimensional atom–diatom reactions and quenching processes; the deviations of the semiclassical results from the accurate quantum mechanical ones are averaged over nine cases. In the adiabatic electronic representation, the FMSM method provides some improvement over Tully’s fewest switches trajectory surface hopping method. However, both methods, irrespective of electronic representation, systematically overpredict the extent of reaction in comparison to the exact quantum mechanical results. The present work provides a baseline for understanding the simplest member of the hierarchy of FMS methods and its relationship to established surface hopping methods.

Generalization of the exponential variational ansatz in relative coordinates for bound state calculations in fourbody systems
View Description Hide DescriptionA generalization of the exponential variational expansion in relative coordinates to fourbody systems is presented. It is shown that all required matrix elements can be expressed as derivatives upon the nonlinear parameters of the socalled auxiliary or basic fourbody (sixdimensional) integral. If such an integral can be computed analytically, then it is possible to use the proposed variational approach for highly accurate variational calculations for various fourbody systems with arbitrary particle masses without adiabatic or similar approximations. For instance, this approach can be used for highly accurate, bound state, variational calculations of the positronium molecule fourbody muonic molecules, and DT molecules and many other systems. In fact, the present work opens a new avenue in the study of fourbody systems.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Direct determination of the spinorbit reactivity in reactions
View Description Hide DescriptionBy exploiting two different Clbeam sources and concurrently monitoring the concentrations of the two reagents and and the H or Datom product, the spinorbit specific excitation functions of the title reactions were determined. The exceptionally large nonadiabatic reactivity for inferred in our previous differential cross section investigation, is now confirmed and quantified. The isotope effects for both the spinorbit ground and excited reagents are also elucidated.

Photodissociation–ionization dynamics of molecular chlorine Rydberg states using velocity map imaging
View Description Hide DescriptionVelocity map images are reported for photoelectrons and atomic chlorine ions produced during onecolor REMPI (resonance enhanced multiphoton ionization) of molecular chlorine via the vibrational levels of the Rydberg state. Previous magnetic bottle photoelectron studies by Koenders et al. of the same process have shown that REMPI proceeds by twophotonresonant excitation, followed by coreexcitation to a superexcited Rydberg state at the threephoton level. This state undergoes (auto)ionization, resulting in the formation of groundstate molecular chlorine ions, and/or dissociation to neutral products, resulting in the production of electronically excited neutral chlorine atoms. Photoelectrons arise from ionization of and electronically excited Cl atoms, while ions arise from the dissociation of and from ionization of the excited Cl atoms. The chlorine ion velocity map images reveal new information on the dissociation–ionization dynamics of superexcited and the dissociationdynamics of the subsequently formed ions. In the latter case an unexpected lowenergy (perpendicular) dissociation pathway to product atoms is observed. Results from the photoelectron images are compared with those from the magnetic bottle studies. While the imaging kineticenergy resolution is less than that of the magnetic bottle spectrometer, the angular distribution information and lack of velocity bias of imaging is advantageous. Most of the trends observed in the electron and images can be rationalized in terms of singleelectron excitation processes and the known molecular orbital structure of the electronic states involved.

Guided ion beam study of collisioninduced dissociation dynamics: integral and differential cross sections
View Description Hide DescriptionThe low energy collisioninduced dissociation (CID) of with Xe is investigated using a recently modified guided ion beam tandem mass spectrometer, in the energy range from 0 to 5 eV in the centerofmass (CM) frame. The additions to the instrument, updated with a double octopole system, and the new experimental methods available are described in detail. Integral cross sections for product formation are presented and analyzed using our standard modeling procedure. A slightly revised value for the bond dissociation energy of of is obtained, in very good agreement with literature values. Axial and radial velocity distributions for primary and product ions are measured at 1.3, 2.0, and 2.7 eV, in the threshold region for product formation. The resulting velocity scattering maps are presented and discussed. Evidence of efficient energy transfer is observed from angular scattering of CID products. Experimental distributions of residual kinetic energies are derived and extend to zero, the point of 100% energy deposition. This indicates that energy transfer is nonimpulsive and probably associated with transient complex formation. For the first time, the experimental residual kinetic energy distributions are compared with the predictions of the empirical model used in integral cross section analyses. Good agreement is observed within experimental uncertainties. A model for the distribution of deposited energy during collisional activation is derived on the basis of these experimental observations.

Ab initio rovibrational spectroscopy of hydrogen sulfide
View Description Hide DescriptionPotential energy hypersurfaces (PES) have been constructed for the ground electronic state of utilizing results from stateoftheart ab initio quantum chemical methods, most notably higherorder coupled cluster theory employing (corepolarized) correlationconsistentbasis sets. Small corrections due to extrapolation to the complete basis set and full configuration interaction limits, core correlation, and relativistic corrections, as well as effects beyond the Born–Oppenheimer approximation have been investigated and incorporated into the final PES. Using the exact rovibrational kinetic energy operator rovibrational energy levels have been computed with the different PESs. The final converged ab initio PES of this study reproduces the available vibrational band origins of and with maximum deviations, gradually increasing for increased vibrational excitation, of 29(14 300), 10(3800), 7(4600), and 12(6400) cm^{−1}, respectively, where the maximum energy above the zeropoint energy is given in parentheses. The errors are considerably larger for the bending states than for the stretching states. Reproduction of rotational term values, given explicitly for of the vibrational ground state, shows remarkable agreement between experiment and the purely ab initio approach of this study.

New inversion coordinate for ammonia: Application to a CCSD(T) bidimensional potential energy surface
View Description Hide DescriptionA new inversion coordinate is defined for ammonia as a function of the valence angles. Its square is similar to the often used totally symmetric bending displacement coordinate for the pyramidal –type molecules. We have used this in a twodimensional calculation including the totally symmetric stretching and the inversion mode. A conventional symmetrized internal coordinate is employed for the symmetric stretch. A twodimensional potential energy surface is calculated using the ab initio CCSD(T) method together with the augccpVTZ, ccpVQZ, and augccpVQZ basis sets. The corresponding eigenvalues are calculated variationally using a Morse oscillator basis set for the stretch and a harmonic oscillator basis set for the inversion. A good agreement is obtained between the calculated and 22 experimental inversion levels, 9 of and the others involving 4 other isotopomers and With the augccpVTZ basis, a mean absolute error of 5.0 is obtained whereas with the augccpVQZ basis set the error becomes 7.9

Theoretical study of the photochemistry of
View Description Hide DescriptionThis is a theoretical study of the photochemistry of based on ab initiopotential energy surfaces and trajectory surfacehopping calculations. We calculated quasidiabatic states and couplings for eight singlet states of with a multireference perturbationconfiguration interaction (CI) technique. Analytical representations of the threedimensional potential energy surfaces and electronic couplings were used for semiclassical simulations of the nonadiabaticdynamics of excited up to 5.4 eV. The computational results allow us to relate the photodissociation mechanism to observable quantities such as the anisotropy of the recoil velocity and the translational energy distribution of the fragments.

Transferability in alkyl monoethers. II. Methyl and methylene fragments
View Description Hide DescriptionThe transferability of the atomic and bondproperties of the methyl and methylene fragments in linear unbranched alkyl monoethers was studied using the Theory of Atoms in Molecules (AIM). This theory has been applied to the analysis of the electron charge distributions of a series of 33 dialkyl ethers, The results obtained indicate that the methyl and methylene fragments situated in α, β, γ, or δ positions with respect to the oxygen atom are different to those of an nalkane. Nevertheless, and at more distant positions can be considered as standard units, whose nonenergetic properties coincide with those of the corresponding fragment in an nalkane. On the contrary, the energetic properties of the fragments maintain a differential value with respect to the nalkane in all of the positions studied in the series. The properties of the methyl or methylene fragments in α to the oxygen depend on the size (methyl or larger) of the other alkyl chain bonded to the oxygen. The properties of methylenes are also different when they are α to the terminal Thus, the and fragments of dialkyl ethers can be classified into and groups. All of the groups proposed verify the transferability of bondproperties,charge, and volume throughout all the fragments that it includes. Though the energy of the fragments depend on the size of the molecule, fragments included in the same group display a common dependence. This dependence does not impede the appearance of excellent linear relationships between the total molecular energy and the number of groups. Nevertheless, the AIM computed energies for the oxygen atom are always more negative than those obtained from the fittings of total electron energies to the number of groups in the molecule. This stabilization is produced at the cost of destabilizing the or groups in the α position. Whereas, if the groups bonded to a methyl group are excluded, the remaining and groups are slightly stabilized (in a magnitude that depends on the size of the molecule and which oscillates slightly in its position with respect to the oxygen atom).

Reaction of acetaldehyde cations with water: The effects of vibrational mode and impact parameter on reactivity and product branching
View Description Hide DescriptionScattering of modeselectively excited acetaldehyde cations from was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured.Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a longlived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly modespecific dependence on vibrational state.

The lowest doublet and quartet potential energy surfaces involved in the reaction. I. Ab initio study of the symmetry abstraction and insertion mechanisms
View Description Hide DescriptionIn this work we have carried out ab initio complete active space selfconsistentfield (CASSCF) calculations, secondorder perturbation calculations based on CASSCF wave functions (CASPT2), uncontracted multireference configuration interaction calculations, and some density functional calculations with standard correlationconsistent Dunning basis sets and atomic natural orbital basis sets on the lowest and potential energy surfaces involved in the title reaction. The ground surface has an average energy barrier of 5.3 kcal/mol in the CASPT2 complete basis set limit. A peroxy NOO minimum is found in agreement with preceding ab initio works, which seems to play an important role in the opening of a double microscopic mechanism: direct abstraction and indirect insertion through the molecule. The ground surface shows an average energy barrier of 13.5 kcal/mol in the CASPT2 complete basis set limit. Despite this excited surface displays another peroxy minimum, in this case only a direct abstraction mechanism can be expected. The present results improve previous high quality ab initio studies and provide lower energy barriers in both potential energy surfaces, which would produce larger total thermal rate constants in better agreement with experimental data. Finally, it is demonstrated that the N and O electron correlation cannot be neglected as it produces a significant decrease in both energy barriers.

The effect of substituents on the vibronic structure of the electronic spectra of α,ωdithienylpolyenes: A computational study
View Description Hide DescriptionWe present a quantum chemical investigation of the vibronic structure in the electronic spectra of three α,ωdithienylpolyenes. Equilibrium structures and vibrational force fields of ground and excited electronic states are computed at semiempirical and ab initio levels of theory. The molecular parameters are employed to calculate displacement parameters for totally symmetric modes which are then used to model the vibronic structure of emission and absorption spectra. The observed spectral features are well reproduced by the computed spectra and two characteristic frequency regions are identified for the emission spectra. The first, above is due to the stretching activity typical of trans polyenes. In addition, the strong activity in emission around is rationalized by considering that the two etherocyclic rings add, to the central polyenic chain, a conjugated carbon skeleton with cis linkages. In analogy with the oligomers of thiophene, the cis linkages, along with vibronic coupling, are responsible for the remarkable activity observed in emission in this frequency region.

The transition of CrH, Einstein coefficients, and an improved description of the state
View Description Hide DescriptionThe spectrum of CrH has been reinvestigated in the region using the Fourier transform spectrometer of the National Solar Observatory. The 10 and 11 bands of the transition have been measured and improved spectroscopic constants have been determined. A value for the 20 band origin has been obtained from the band head using estimated spectroscopic constants. These data provide a set of much improved equilibrium vibrational and rotational constants for the state. An accurate description of the transition has been obtained using a multireference configuration interaction approach. The inclusion of both scalar relativity and Cr correlation are required to obtain a good description of both states. The ab initio computed Einstein coefficients and radiative lifetimes are reported.

Photoabsorption of NaCl clusters at the Na Kedge: Development of the bond length with the cluster size
View Description Hide DescriptionPhotoabsorptionspectra of free and deposited NaCl clusters at the Na Kedge are presented. The absorption spectra were recorded using the yield of photoelectrons and cluster ions. Above the Kshell ionization limit, multiscatteringtype oscillations could be identified which allow the determination of bond length in the cluster. The variation of the bond length with cluster size is in reasonable agreement with model calculations, assuming a Coulomb interaction and a Born–Meyer type repulsion of the ions.