bluuebunny/crossref_metadata_2025 huggingface dataset:
The mechanism of the unimolecular dissociation of trichloroethylene CHCl=CCl2 in the ground electronic state
Authors: Yokoyama Keiichi , Fujisawa Ginji , Yokoyama Atsushi
Published: July 2002

The unimolecular dissociation of trichloroethylene in its electronic ground state has been investigated using an infrared multiphoton dissociation combined with photofragmentation translational spectroscopy to measure product translational energies. The main reaction channel was found to be HCl elimination on the basis of observed product time-of-flight (TOF) spectra. A center-of-mass translational energy distribution for this channel provides direct evidence for competition between two channels, three- and four-centered HCl eliminations. Cl elimination was found to be a minor but significant channel from observed Cl+ and C2HCl+TOF spectra. The branching ratios were determined as 0.28, 0.55, and 0.17 for the three- and four-centered HCl eliminations and the Cl elimination, respectively. The three-centered channel exhibits a ‘‘statistical’’ translational energy distribution which is typical for a reaction with no potential energy barrier in the reverse reaction, that is to say, no exit barrier reaction. In contrast, the four-centered channel exhibits a ‘‘nonstatistical’’ translational energy distribution having a peak at around 2 kcal/mol in energy, indicating that a significant exit barrier exists in the channel. The fraction of potential energy converted to translational energy was estimated to be around 10%. Ab initio calculations at the QCISD(T)/6-311+G**//MP2(FC)/6-31G* level were employed to confirm the reaction mechanism. The agreement in the energetics is quite good.
Application of gradient-corrected density functional theory to the structures and thermochemistries of ScF3, TiF4, VF5, and CrF6
Authors: Russo Thomas V. , Martin Richard L. , Jeffrey Hay P.
Published: July 2002

Density functional theory (DFT) and Hartree–Fock (HF) calculations are reported for the family of transition metal fluorides ScF3, TiF4, VF5, and CrF6. Both HF and the local-density approximation (LDA) yield excellent agreement with experimental bond lengths, while the B-LYP gradient-corrected density functional gives bond lengths 0.04−0.05 Å too long. An investigation of various combinations of exchange and correlation functionals shows that, for this series, the origin of this behavior lies in the Becke exchange functional. Much improved bond distances are found using the hybrid HF/DFT functional advocated by Becke. This approximation also leads to much improved thermochemistries. The LDA overestimates average bond energies in this series by 30−40 kcal/mol, whereas the B-LYP functional overbinds by only ∼8−12 kcal/mol, and the hybrid HF/DFT method overbinds by only ∼2 kcal/mol. The hybrid method predicts the octahedral isomer of CrF6 to be more stable than the trigonal prismatic form by 14 kcal/mol. Comparison of theoretical vibrational frequencies with experiment supports the assignment of an octahedral geometry.
Incorporating backflow into a relaxation theory treatment of the dynamics of nonequilibrium nonadiabatic transition processes
Authors: Evans Deborah G. , Coalson Rob D.
Published: July 2002

An approximate method for computing the leakage of population from an initial (‘‘donor’’) electronic state, prepared in a nonequilibrium nuclear coordinate distribution, onto a second, nonadiabatically coupled (‘‘acceptor’’), electronic state is presented. This proposed solution, which utilizes a set of coupled integrodifferential equations (commonly referred to as Generalized Master Equations) is an extension of a nonequilibrium golden rule formula derived previously [R. D. Coalson, D. G. Evans, and A. Nitzan, J. Chem. Phys. 101, 486 (1994)]. The Generalized Master Equation approach is able to describe situations where the donor and acceptor potential energy surfaces have similar energy origins, and hence irreversible flow of population from the donor state to the acceptor state is not expected. The accuracy of the method is demonstrated for an exactly solvable spin–boson model of inner sphere electron transfer. In the regime of small nonadiabatic coupling, agreement of the proposed method and path integral calculations is nearly quantitative for symmetric electron transfer processes and systems with weak bias between the energy origins of the donor and acceptor electronic states. Unlike the nonequilibrium golden rule formula, appropriately constructed Generalized Master Equations are capable of capturing the backflow of electronic population from the acceptor to the donor surface and relaxation to Boltzmann equilibrium at long times.
The performance of the explicitly correlated coupled cluster method. I. The four-electron systems Be, Li−, and LiH
Authors: Noga Jozef , Tunega Daniel , Klopper Wim , Kutzelnigg Werner
Published: July 2002

To study the reliability of the recently developed explicitly correlated coupled cluster method (CCSDT1-R12) we have performed calculations on the four-electron systems Be, Li−, and LiH on various levels of perturbation theory and coupled cluster theory with and without explicit linear rij terms. The convergence of the total energy with increasing size of the basis is much faster than for the conventional coupled cluster approach. Our CCSDT1-R12 energies of −14.667261 Eh for Be and −7.500671 Eh for the Li− ground state are the best ones computed so far and are close to previous estimates of the CCSDT 1 basis set limits. The Be result differs from the ‘‘experimental’’ nonrelativistic energy by ca. 0.1 mEh, mainly due to neglect of quadruple excitations. Our Born–Oppenheimer energy of LiH at the equilibrium distance of −8.070487 Eh is close to the experimental nonrelativistic energy. The binding energy (D0) of LiH with respect to Li+ and H− is calculated as −7.152 eV, in agreement with the experimental value within a meV. For LiH the harmonic vibrational frequencies and other related spectroscopic constants are studied in their basis dependence as well. The equilibrium distance and the harmonic vibrational frequency of LiH are much less sensitive to the inclusion of terms that explicitly depend on the interelectronic coordinates. Basis set superposition errors are much smaller in the R12 approach than in the conventional calculations, especially for the smaller basis sets.
Self-consistent intermediate Hamiltonians: A coupled cluster type formulation of the singles and doubles configuration interaction matrix dressing
Authors: Nebot-Gil I. , Sánchez-Marín J. , Malrieu J. P. , Heully J. L. , Maynau D.
Published: July 2002

This paper presents a new self-consistent dressing of a singles and doubles configuration interaction matrix which insures size-consistency, separability into closed-shell subsystems if localized molecular orbitals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coupled cluster method, including fully the cluster operators of single and double excitations, and partially those of the triples (Bartlett’s algorithm named CCSDT-1a). Further improvement can be easily included by adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consistent combination with less expensive treatments for the study of very large systems.
Mode-specific hydrogen tunneling in tropolone: An instanton approach
Authors: Smedarchina Zorka , Siebrand Willem , Zgierski Marek Z.
Published: July 2002

Calculations are reported of hydrogen and deuterium tunneling splittings in the ground state S0 (X̃,1A1) and the first excited singlet state S1 (Ã,1B2) of tropolone-d0 and -d1. The main focus of the calculations is on the splittings observed in vibrationally excited levels of S1, some of which are larger while others are smaller than the zero-point splitting. To account for these observations, a potential-energy surface is constructed by standard quantum-chemical methods and the dynamics on this surface is treated by a method derived from the instanton approach. The potential-energy surface is a complete multidimensional surface resulting from the combination of a potential-energy curve along the tunneling coordinate with a harmonic force field calculated at the stationary points. The level of calculation adopted is HF/6-31G** for S0 and CIS/6-31G** for S1. A few other, nominally more accurate, methods were tried but proved to be unsatisfactory. To deal with the dynamics, the instanton method, used previously for the calculation of zero-point level splittings, is modified so as to make it applicable to excited levels. As expected, it is found that excitation of the tunneling mode strongly promotes hydrogen transfer. The effects of exciting modes that are symmetric or antisymmetric with respect to the symmetric transition state are evaluated for all such modes with assigned splittings by a straightforward generalization of the correction terms previously derived for zero-point splittings. Of special interest are out-of-plane modes, some of which show up as overtones with splittings smaller than the zero-point splitting, despite the fact that there is no linear coupling between these modes and the tunneling mode. The effect is ascribed to anharmonic coupling and an effort is made to calculate the required anharmonicities quantum-chemically. In general the agreement between theory and experiment is satisfactory for modes that are linearly coupled while the situation is less clear for anharmonically coupled modes.
Ab initio study of the adducts of carbon monoxide with alkaline cations
Authors: Ferrari A. M. , Ugliengo P. , Garrone E.
Published: July 2002

The interaction between CO (either via the C or the O end) and the alkaline cations (Li+, Na+, K+, Rb+, and Cs+) has been studied by means of six ab initio methods, featuring the classical Hartree–Fock, the second order Mo/ller–Plesset treatment of electron correlation, one local density functional and two gradient-corrected methods as well as a quadratic configuration interaction inclusive of single and double substitutions with a noniterative triples contribution to the energy. Basis sets adopted for CO, Li+, Na+, and K+ and the corresponding adducts are of triple-ζ valence quality augmented with a double set of polarization functions (d on C and O; p on the cations). For Rb+ and Cs+, Hay–Wadt effective core potential basis sets have been adopted. Calculated features are the binding energy, the frequency and intensity of the CO stretch, the bending mode, the cation-carbon (or oxygen) stretch, and the equilibrium geometry. Gradient-corrected density functional methods yield results nearly as good as the most expensive correlated method based on configurations interaction. A number of correlations are established among the observables. The role of electrostatics in the interaction is analyzed both by studying the molecular electrostatic potential of CO and by replacing the cation with a proton in the same position. Binding through the C end is invariably preferred, though, with increasing size of the cation, binding through the O end become progressively less unfavored. Experimental data concerning alkaline-cation substituted zeolites are compared with computational results, and an overall agreement is observed.
Resonant two-photon ionization spectra of the van der Waals complexes: C6H5X⋅⋅⋅N2 (X=F, Cl, Br)
Authors: Hu Yihua , Lu Wenyun , Yang Shihe
Published: July 2002

The one-color resonant two-photon ionization technique is employed to study jet-cooled van der Waals (vdW) clusters of halobenzene and nitrogen through the S0→S1 transition around the 0̄00 band. The spectra obtained exhibit rich information about the clusters’ intermolecular vdW vibrational modes and their solvent internal rotation. We have tentatively assigned all the observed spectral features to a single isomer of C6H5X⋅⋅⋅N2 (X=F, Cl, Br). The influence of X on the vdW vibrations and the internal rotation of N2 in the complex is investigated. The analysis of the influence of X on the 0̄00 spectral shift suggests that the change in vdW interaction energy upon electronic excitation is mainly due to the dispersion term.
Coadsorption of hydrogen with ethylene and acetylene on Si(100)-(2×1)
Authors: Widdra W. , Huang C. , Yi S. I. , Weinberg W. H.
Published: July 2002

The adsorption, desorption, and thermal decomposition of acetylene and ethylene on the Si(100)-(2×1) surface have been investigated with emphasis on the modifications induced by coadsorbed hydrogen. Based on high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption spectroscopy (TPD), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES), we show that the adsorption of acetylene and ethylene is blocked by preadsorbed hydrogen leading to a hydrocarbon saturation coverage which decreases linearly with hydrogen precoverage. At low temperatures preadsorbed hydrogen has no influence on the surface chemistry of acetylene or ethylene. At approximately 550 K, coadsorbed hydrogen induces decomposition of ethylene which is not observed in the absence of hydrogen. After postexposures of an ethylene-saturated Si(100)-(2×1) surface to gas-phase atomic hydrogen with fluences below 5×1014 cm−2, the ethylene is essentially unperturbed at low surface temperatures with partial decomposition upon heating as for preadsorbed hydrogen. Higher postexposures of atomic hydrogen lead to Si–C bond cleavage and the formation of ethyl. The desorption of molecular ethylene is then up shifted by approximately 100 K. The experimental results and observed reaction intermediate are explained by an elemental adsorption and reaction model.
Site-selective participator decay of core-excited butadiene
Authors: Sorensen S. L. , Osborne S. J. , Ausmees A. , Kikas A. , Correia N. , Svensson S. , Naves de Brito A. , Persson P. , Lunell S.
Published: July 2002

The decay of core-excited electronic states in free 1,3 trans butadiene molecules has been studied using high-resolution synchrotron radiation and electron spectrometry. The core-level energy shift between the terminal and central carbon atoms is 0.64 eV making selective excitation of core electrons from these atoms possible. Resonant excitation to the au(π*) valence state leads to autoionizing decay channels which proceed according to the atomic site in the molecule. The radiationless decay is localized, and certain molecular orbitals are excluded from the decay depending upon the site of the core hole. This phenomenon is confirmed by semiempirical INDO calculations based upon the equivalent core approximation. The vibrational structure of the resonances below the carbon K edge has been measured and fit to extract vibrational energies and intensities, chemical shifts, and the lifetimes of the centrally and terminally excited states. The C 1s spectrum is also measured with vibrational resolution and the energies of the normal vibrational modes are extracted. The bond lengths are derived by application of a linear coupling analysis.
Molecular dynamics of rigid molecules
Authors: Bulgac Aurel , Adamuţi-Trache Maria
Published: July 2002

We develop a Hamiltonian formalism for an ensemble of rigid molecules in the quaternionic representation. In the quaternionic language different symmetries of the rigid top dynamics acquire a simple and natural expression. Subsequently we describe the coupling of the ensemble of rigid molecules to a thermostat. The isothermal molecular dynamics is defined by introducing additional pseudofriction coefficients, according to a generalized Nosé–Hoover prescription.
Multipolar polarizabilities and two- and three-body dispersion coefficients for alkali isoelectronic sequences
Authors: Patil S. H. , Tang K. T.
Published: July 2002

Using simple wave functions based on the asymptotic behavior and on the binding energies of the valence electron, we have evaluated multipolar matrix elements. They allow us to obtain polarizabilities up to α12 of Li, Na, K, Rb, Cs, Be+, Mg+, Ca+, Sr+, Ba+, and dispersion coefficients of homonuclear and heteronuclear interactions from c6 to c24. Comparisons with previously determined low order quantities show that this approach is capable of yielding quite useful values for these quantities.
Scattering-angle resolved product rotational alignment for the reaction of Cl with vibrationally excited methane
Authors: Orr-Ewing A. J. , Simpson W. R. , Rakitzis T. P. , Kandel S. A. , Zare R. N.
Published: July 2002

We have applied the experimental technique of core extraction [W. R. Simpson et al., J. Chem. Phys. 103, 7299 (1995)] combined with resonance-enhanced multiphoton ionization (REMPI) with a polarized laser beam to probe the angular-momentum alignment of the HCl product of the reaction of Cl with vibrationally excited CH4(ν3=1). The core extraction method permits us to distinguish products scattered in different directions in the center-of-mass frame, and thus we are able to determine the rotational alignment for various product scattering angles for individual HCl(v,J) quantum states (a state-resolved three-vector correlation). For the forward-scattered HCl(v=1, J=1) we observe a large positive rotational alignment. This positive velocity-angular-momentum correlation is interpreted to be the result of the angular momentum of the HCl product being directed in the plane perpendicular to the line-of-centers force in a simple hard-sphere scattering model.
The chemistry of the superheavy elements. I. Pseudopotentials for 111 and 112 and relativistic coupled cluster calculations for (112)H+, (112)F2, and (112)F4
Authors: Seth Michael , Schwerdtfeger Peter , Dolg Michael
Published: July 2002

One- and two-component (spin–orbit coupled) relativistic and nonrelativistic energy adjusted pseudopotentials and basis sets for the elements 111 and 112 are presented. Calculations on the positively charged monohydride of the recently discovered superheavy element 112 are reported. Electron correlation is treated at the multireference configuration interaction and coupled cluster level and fine structure effects are derived from a single-reference configuration interaction treatment. Relativistic effects decrease the (112)H+ bond distance by 0.41 Å. This bond contraction is similar to the one calculated recently for (111)H [Chem. Phys. Lett. 250, 461 (1996)]. As a result the bond distance of (112)H+ (1.52 Å) is predicted to be smaller compared to those of the hydrides of the lighter congeners HgH+ (1.59 Å), CdH+ (1.60 Å) and similar to that of ZnH+ (1.52 Å). We predict that (112)H+ is the most stable hydride in the Group 12 series due to relativistic effects. As in the case of (111)H the relativistic increase of the stretching force constant is quite large, from 1.5 to 4.3 mdyn/Å at the coupled cluster level. The trend in the dipole polarizabilities of the Group 12 elements is discussed. Relativistic and electron correlation effects are nonadditive and due to the relativistic ns contraction (n=7 for 112), correlation effects out of the (n−1)d core are more important at the relativistic than the nonrelativistic level. We also show evidence that element 112 behaves like a typical transition element, and as a consequence the high oxida-tion state +4 in element 112 might be accessible.
Laser cooling of internal degrees of freedom. II
Authors: Bartana Allon , Kosloff Ronnie , Tannor David J.
Published: July 2002

Theoretical progress in the cooling of internal degrees of freedom of molecules using shaped laser pulses is reported. The emphasis is on general concepts and universal constraints. Several alternative definitions of cooling are considered, including reduction of the von Neumann entropy, −tr{ρ̂logρ̂} and increase of the Renyi entropy, tr{ρ̂2}. A distinction between intensive and extensive considerations is used to analyse the cooling process in open systems. It is shown that the Renyi entropy increase is consistent with an increase in the system phase space density and an increase in the absolute population in the ground state. The limitations on cooling processes imposed by Hamiltonian generated unitary transformations are analyzed. For a single mode system with a ground and excited electronic surfaces driven by an external field it is shown that it is impossible to increase the ground state population beyond its initial value. A numerical example based on optimal control theory demonstrates this result. For this model only intensive cooling is possible which can be classified as evaporative cooling. To overcome this constraint, a single bath degree of freedom is added to the model. This allows a heat pump mechanism in which entropy is pumped by the radiation from the primary degree of freedom to the bath mode, resulting in extensive cooling.
Photodissociation spectroscopy and dynamics of the HCCO free radical
Authors: Osborn David L. , Mordaunt David H. , Choi Hyeon , Bise Ryan T. , Neumark Daniel M. , McMichael Rohlfing Celeste
Published: July 2002

The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B̃ 2Π←X̃ 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an improved heat of formation for HCCO, ΔHf,2980=1.83±0.03 eV.
Linear and nonlinear polarizabilities of polydiacetylene and polybutatriene chains: An ab initio coupled Hartree–Fock investigation
Authors: Perpète Eric A. , Champagne Benoı̂t , Kirtman Bernard
Published: July 2002

Ab initio Hartree–Fock 6-31G longitudinal polarizabilities and second hyperpolarizabilities of polydiacetylene and polybutatriene oligomers are computed and, then, extrapolated to the infinite polymer limit. Both the electronic and vibrational components are obtained with the latter evaluated in the double harmonic approximation. For static fields the electronic hyperpolarizability of the butatrienic form is almost 50 times larger than the acetylenic form; the ratio of the vibrational to the electronic component is 0.86 for the latter and 3.3 for the former. Thus, the two components are sensitive to the difference in bond length alternation (BLA) in different ways. Depending upon the particular nonlinear process it is shown that the vibrational hyperpolarizability may remain large even at optical frequencies. Three intense Raman-active k=0 modes dominate the vibrational component. At the level of theory used here these modes occur in the frequency range 1000–2500 cm−1 and they consist of changes in the BLA as well as hydrogen wagging motions. For polydiacetylene, there is a 1:1 relation with the corresponding vibrations that make the most important hyperpolarizability contributions in polyacetylene and polyyne.
Microwave spectrum of the MnO radical in the X 6Σ+ state
Authors: Namiki Kei-ichi , Saito Shuji
Published: July 2002

The microwave spectrum of MnO in the Σ+6 ground electronic state was detected using a source-modulated submillimeter-wave spectrometer. The MnO radical was efficiently generated by dc sputtering of manganese flakes placed inside a hollow cathode in the presence of an oxygen and helium mixture. In total, 283 spectral lines were measured in the frequency region of 210–450 GHz for nine rotational transitions, each of which showed six fine structure line groups consisting of several hyperfine structure components due to the Mn55 nucleus (I=5/2). A least-squares analysis of the measured line frequencies resulted in the determination of rotational, fine, and hyperfine coupling constants including higher-order spin–orbit distortion terms for the spin–spin, spin–rotation interactions and the Fermi contact interaction of the Mn nucleus. The hyperfine coupling constants were used to assess plausible molecular orbital bonding models.
Spin-restricted open-shell coupled-cluster theory
Authors: Szalay Péter G. , Gauss Jürgen
Published: July 2002

Spin-restricted CC theory is suggested as a new approach for the treatment of high-spin open-shell systems in CC theory. Spin constraints are imposed on the wave function in the sense that the projected spin eigenvalue equations are fulfilled within the (truncated) excitation space. These constraints allow a reduction in the number of independent amplitudes, thus decreasing the computational cost when implemented efficiently. The approach ensures that the spin expectation value always corresponds to the exact value, though the wave function is (for truncated CC treatments) not rigorously spin-adapted. For the specific case of high-spin doublets, detailed equations are derived for amplitudes, energy and first derivatives of the energy within the computationally most useful singles and doubles approximation. Numerical examples demonstrate the excellent performance of the spin-restricted CC approach relative to the more traditional spin-orbital based CC treatments, suggesting that it might be an attractive alternative for the treatment of high-spin open-shell systems.
Novel pseudoensembles for simulation of multicomponent phase equilibria
Authors: Escobedo Fernando A.
Published: July 2002

Pseudoensembles are formulated to conduct Monte Carlo simulations of phase coexistence in multicomponent systems. It is shown that this approach can be applied to different types of ensembles. In contrast to the conventional Gibbs ensemble that performs flash-type calculations, pseudoensembles can also be designed to perform bubble-point and dew-point calculations. It is also shown that pseudoensemble simulations provide some connections between histogram reweighting techniques, Gibbs ensemble simulations, Gibbs–Duhem integrations, and thermodynamic integrations. In particular, pseudoensembles naturally lead to the formulation of Gibbs–Duhem integrations. The methods proposed here are validated by simulating vapor-liquid phase equilibrium of model binary mixtures.
Femtosecond study of photo-induced electron dynamics in AgI and core/shell structured AgI/Ag2S and AgBr/Ag2S colloidal nanoparticles
Authors: Brelle Michael C. , Zhang Jin Z.
Published: July 2002

Direct measurements of the dynamics of photoinduced electrons in AgI, AgI/Ag2S, and AgBr/Ag2S semiconductor colloidal nanoparticles have been performed using femtosecond pump–probe laser spectroscopy. In AgI the transient absorption signal features a laser pulsewidth limited rise followed by a double exponential decay with time constants of 2.5 ps and >0.5 ns. The decay dynamics were found to be independent of pump power, indicating that the decays are not second order kinetic processes. The dynamics were also independent of the probe wavelengths, suggesting that the absorption spectrum of the photogenerated electrons is fully developed within ∼150 fs. The fast 2.5 ps decay is attributed to trapping and nonradiative electron–hole recombination mediated by a high density of trap states while the slow decay is attributed to reaction of deep trapped electrons with silver ions to form Ag atoms. For core/shell structured cocolloids of AgI/Ag2S, similar decay dynamics were observed. The only interesting difference is that the fast decay becomes faster with increasing concentration of Ag2S. This is due to direct excitation of Ag2S, which has a faster initial decay (800 fs) than in AgI. This is supported by the results of AgBr/Ag2S for which the same 800 fs decay was observed while no signal was observed for AgBr colloids alone, indicating clearly that the signal is from Ag2S. These findings on the early time dynamics of photo-induced excitons in AgI, AgI/Ag2S, and AgBr/Ag2S are important in better understanding the photographic process involving silver halides.
Coupled-cluster calculations of the electronic excitation spectrum of free base porphin in a polarized basis
Authors: Gwaltney Steven R. , Bartlett Rodney J.
Published: July 2002

Similarity transformed equation-of-motion coupled-cluster calculations on the excited states of free base porphin are reported. These are the first calculations to use polarization functions for all of the excited states, and the polarization functions are found to be very important for accurately describing the optically allowed states. These calculations strongly support the traditional interpretation of the electronic spectrum of free base porphin.
Monte Carlo simulations of symmetric polymer blends. II. Chain properties in interface regions
Authors: Olaj Oskar Friedrich , Petrik Thomas , Zifferer Gerhard
Published: July 2002

Making use of a previously introduced lattice model for binary polymer blends of limited miscibility Monte Carlo simulations have been performed in order to investigate the influence of different thermodynamic conditions on mass segregation and global structure as well as on microstructure of homopolymers when passing from one bulk phase through the interface to the other. The calculations show an enrichment of shorter chains at the interface as well as a decreasing miscibility with decreasing average chain length (at constant bulk density) of the polymer matrix on the one hand and relate the compression of the coils at the interface to the observed orientation and deformation effects on the other. Surprisingly, an additional expansion of the polymer coils (belonging to the minority component) can be found at some distance from the actual interface due to attractive interactions between pairs of chains in rather “diluted” layers. Furthermore, all phenomena are discussed from two different viewpoints: (i) The average chain length is kept constant while the energy parameter is varied, and (ii) the chain length is varied and the bulk densities are held constant by use of identical relative energy parameters.
Efficient sampling of solvent free energies in polymers
Authors: van der Vegt N. F. A. , Briels W. J.
Published: July 2002

Solvent excess chemical potentials in amorphous polymers were calculated by means of molecular dynamics simulations using two different methods, the thermodynamic integration method and an extended ensemble method, during which the coupling of a test particle with the rest of the system is continuously changing. The latter method was found to probe the simulation box very efficiently whereas, in the thermodynamic integration, only limited regions of the box were sampled. Excess free energies of chloroform in swollen polydimethylsiloxane were found to decrease with increasing solvent swelling in qualitative agreement with experiment.
Voids and clusters in expanded water
Authors: Mountain Raymond D.
Published: July 2002

Molecular-dynamics simulations of water at elevated temperatures and reduced densities relative to ambient conditions are used to examine the size distributions of suitably defined hydrogen bonded clusters, physical clusters, and voids. The cluster structure of both hydrogen bonded clusters and physical clusters changes rapidly as a function of density for near critical and supercritical temperatures. The density variation of an observable quantity, a local density, is shown to be a signature of the onset in the change in cluster size. The densities with a broad range of cluster sizes are roughly located in the supercritical temperature region.
Sum-over-states calculation of the nuclear spin–spin coupling constants
Authors: Bouř P. , Buděšı́nský M.
Published: July 2002

Nuclear spin–spin coupling constants calculated using the sum-over-states (SOS) expansions were compared to experimental values and usual coupled-perturbed (CP) calculations. Rigid Kohn–Sham orbitals obtained from a hybrid density functional were used in the SOS model. Its accuracy for small molecules is comparable with the CP results, nevertheless calculated constants were uniformly underestimated. However, the SOS scheme is less limited by molecular size and can be applied for bigger systems than the CP method, as documented on the proton–proton coupling constants in α-pinene, β-pinene, and camphor molecules.
Photodissociation of bromobenzene at 266 nm
Authors: Zhang Hong , Zhu Rong-Shun , Wang Guang-Jun , Han Ke-Li , He Guo-Zhong , Lou Nan-Quan
Published: July 2002

The photodissociation of C6H5Br at 266 nm has been investigated on the universal crossed molecular beam machine, and time-of-flight spectra as well as the angular distribution of Br atom have been measured. Photofragment translational energy distribution P(Et) reveals that about 47% of the available energy is partitioned into translational energy. The anisotropy parameter β at this wavelength is −0.7±0.2. From P(Et) and β, we deduce that C6H5Br photodissociation is a fast process and the transition dipole moment is almost perpendicular to the C–Br bond. Ab initio calculations have been performed, and the calculated results show that the geometry of the first excited state of bromobenzene has changed apparently compared with that of the ground state. Two kinds of possible fast dissociation mechanism have also been proposed.
Monte Carlo study of the capacitance of the double layer in a model molten salt
Authors: Boda Dezső , Henderson Douglas , Chan Kwong-Yu
Published: July 2002

Monte Carlo simulations are reported for charged hard spheres at high density near a charged wall. This system is a simple model for a molten salt double layer. Unfortunately, the reduced temperatures that correspond to experiment are very small. This results in a large Boltzmann factor. As a result, we are unable to obtain meaningful results for such low values and report results only for moderately low values of the reduced temperature. Even so, our results should be a useful benchmark. Further, we are able to give a qualitative answer to an interesting question. We find that at low temperatures the capacitance near the point of zero charge increases with increasing temperature. This agrees with experiment for molten salts and disagrees with the behavior of double layer in dissolved salts, which can be modeled with low density and high temperature charged hard spheres near a wall. This also disagrees with the predictions of the Gouy–Chapman theory and the mean spherical approximation. It appears that it is the approximations, and not the charged hard sphere double layer model, that are at fault for describing double layers in molten salts.
A perturbative density functional theory of inhomogeneous fluid mixture
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

A simple perturbative density functional approach has been developed for an inhomogeneous fluid mixture based on a functional Taylor expansion of the free energy in terms of the density inhomogeneity up to third order. A new approximation for the three-body correlation function for the mixture is proposed and the theory is applied to predict the structure of hard-sphere mixture near a hard wall. The calculated density profiles for both the components are found to agree quite well with the corresponding computer simulation results. This theory for an inhomogeneous fluid is also applied to study the structure of a homogeneous hard-sphere mixture, for which the calculated radial distribution functions are found to compare well with the corresponding results obtained from the Fourier transform of the structure factors from the Percus–Yevick theory of liquid mixtures.
Gaussian geminals in explicitly correlated coupled cluster theory including single and double excitations
Authors: Bukowski Robert , Jeziorski Bogumił , Szalewicz Krzysztof
Published: July 2002

The coupled cluster method with single and double excitations has been formulated in a basis set independent language of first quantization. In this formulation the excitation operators are represented in terms of one- and two-electron cluster functions satisfying a set of integrodifferential equations and the strong orthogonality conditions. These equations are solved iteratively by minimizing appropriate Hylleraas-type functionals. During the iteration process correlation energies of up to fourth order in the Mo/ller–Plesset perturbation operator are extracted. A slight modification of the coupled cluster equations leads to an explicitly correlated formulation of the configuration interaction theory. The method was tested in applications to two- and four-electron systems: He, Li+, H2, Be, Li−, and LiH. The two-electron cluster functions were expanded using explicitly correlated Gaussian geminal bases optimized in the lowest order of perturbation theory. Most of the correlation energies computed at various levels of the coupled cluster and perturbation theory represent the most accurate values to date.
On the accurate theoretical determination of the static hyperpolarizability of trans-butadiene
Authors: Maroulis George
Published: July 2002

Finite-field many-body perturbation theory and coupled cluster calculations are reported for the static second dipole hyperpolarizability γαβγδ of trans-butadiene. A very large basis set of [9s6p4d1f/6s3p1d] size (336 contracted Gaussian-type functions) should lead to self-consistent field (SCF) values of near-Hartree–Fock quality. We report γxxxx=6.19, γxxxz=−0.44, γxxyy=3.42, γzzxx=2.07, γxyyz=−0.50, γxzzz=1.73, γyyyy=14.72, γyyzz=8.46, γzzzz=24.10 and γ̄=14.58 for 10−3×γαβγδ/e4a04Eh−3 at the experimental geometry (molecule on the xz plane with z as the main axis). γ̄=(14.6±0.4)×103e4a04Eh−3 should be a very reliable estimate of the Hartree–Fock limit of the mean hyperpolarizability. Keeping all other molecular geometry parameters constant, we find that near the Hartree–Fock limit the mean hyperpolarizability varies with the C=C bond length as 10−3×γ̄(RC=C)/e4a04Eh−3=14.93+31.78ΔR+30.88ΔR2−2.96ΔR3 and with the C–C bond length as 10−3×γ̄(RC–C)/e4a04Eh−3=14.93−7.20ΔR+3.04ΔR2, where ΔR/a0 is the displacement from the respective experimental value. The dependence of the components of γαβγδ on the molecular geometry parameters is not uniform. Electron correlation corrections have been calculated at various molecular geometries at the coupled-cluster single, double and perturbatively linked triple excitations level of theory for all independent components of γαβγδ. In absolute terms, electron correlation affects strongly the γzzzz, less strongly the γxxxx, and even less strongly the out-of-plane component γyyyy. The present analysis suggests a conservative estimate of (3.0±0.6)×103e4a04Eh−3 for the electron correlation correction to γ̄ at the experimental molecular geometry. Most of this value is appropriate to γzzzz. A static limit of γ̄=(17.6±1.0)×103e4a04Eh−3 is advanced (neglecting vibrational averaging). Even if a crude theoretical estimate of the dispersion of γ̄ at 1064 nm is added to this value, the result sets up an unambiguous claim to accord with the experimental value of (20.18±0.11)×103e4a04Eh−3 [D. P. Shelton, Phys. Rev. A 42, 2578 (1990)].
Calculation of frequency-dependent second hyperpolarizabilities for electric field induced second harmonic generation in the second-order Mo/ller–Plesset perturbation theory
Authors: Kobayashi Takao , Sasagane Kotoku , Aiga Fumihiko , Yamaguchi Kizashi
Published: July 2002

The calculations of frequency-dependent second hyperpolarizabilities for the (static) electric field induced second harmonic generation (ESHG) [γ(−2ω;ω,ω,0)] of the FH, H2O, CO, and NH3 molecules in the second-order Mo/ller–Plesset perturbation theory (MP2) based on the quasienergy derivative (QED) method are performed by numerical differentiation of frequency-dependent first hyperpolarizabilities for the second harmonic generation (SHG) [β(−2ω;ω,ω)] with respect to static electric fields. The QED-MP2 values of the ESHG second hyperpolarizabilities γ∥(−2ω;ω,ω,0) of the H2O, CO, and NH3 molecules show good agreement with the experimental values.
Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0–38) in the energy range of 12.05–18.15 eV
Authors: Song Y. , Evans M. , Ng C. Y. , Hsu C.-W. , Jarvis G. K.
Published: July 2002

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 12.05–18.15 eV, covering ionization transitions O2+(X 2Π1/2,3/2g, v+=0–38,J+)←O2(X 3Σg−, v+=0,N″). While the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=3–5, 9, 11, 12, 22, and 25–38) reported here are the first rotational-resolved photoelectron measurements, the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=25–38) represent the first rotationally resolved spectroscopic data for these states. The simulation of spectra obtained at rotational temperatures of ≈20 and 220 K allows the unambiguous identification of O2+(X 2Π1/2,3/2g, v+⩾21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O2+(A 2Πu, v+=0–12) and O2+(a 4Πu, v+=0–18). Combined with spectroscopic data obtained in the previous emission study and the present PFI-PE experiment, we have obtained accurate Dunham-type expansion coefficients for ionization energies, vibrational constants, rotational constants, and spin–orbit splitting constants covering the O2+(X 2Π1/2,3/2g, v+=0–38) states. Significant local intensity enhancements due to near-resonant autoionization were observed in PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=0–14). The energy region of these states is known to manifest a high density of very strong autoionizing low-n-Rydberg states. The observation of a long PFI-PE vibrational progression with a relatively smooth band intensity profile is also in accord with the direct excitation model for the production of highly vibrationally excited O2+(X 2Π1/2,3/2g) states in the Franck–Condon gap region. Since this experiment was carried out under relatively high rotational temperatures for O2, the PFI-PE data reveal higher rotational transitions and numerous local intensity enhancements, which were not observed in previous vacuum ultraviolet laser studies using a cold O2 molecular beam. The rotational branches found here indicate that photoelectrons are formed predominantly in continuum states with orbital angular momenta l=1,3, and 5.
Nonlinear thermal response at the glass transition
Authors: Schick C. , Merzlyakov M. , Hensel A.
Published: July 2002

In linear response, the complex heat capacity is equal to the entropy compliance divided by temperature. The fluctuation dissipation theorem gives a relation to the correlation function (time domain) or spectral density (frequency domain) of entropy fluctuations. Another formal description of frequency dependent heat capacity at the glass transition is based on the Tool–Narayanaswamy–Moynihan approach of structural recovery. In the limit of small temperature perturbations both models are able to describe measured complex heat capacity. We have measured the response at medium temperature amplitudes, when first nonlinearities appear, and have compared the results with the behavior expected from both approaches. We have studied higher harmonics in the temperature amplitude range 0.1–15 K. At the glass transition of poly(vinyl acetate) (PVAc) we observe 1% of higher harmonics at temperature amplitudes of 1 K. Even harmonics can be described quantitatively by the temperature dependence of first harmonic (heat capacity) up to temperature amplitudes of 6 K. This is not possible for odd harmonics. The remaining small odd harmonics support, on a qualitative basis, the fluctuation approach to glass transition.
Property calculations using perturbed orbitals via state-specific multireference coupled-cluster and perturbation theories
Authors: Chattopadhyay Sudip , Mahapatra Uttam Sinha , Mukherjee Debashis
Published: July 2002

In this paper we apply the recently developed state-specific multireference coupled-cluster and perturbation theories to calculate electrical properties such as dipole moment and static polarizability using perturbed orbitals in finite fields. The theories are built on complete active space reference functions, and are designed to treat quasidegeneracy of varying degrees while bypassing the intruder problem. Numerical results are presented for the model systems H4 with trapezoidal geometry and the lowest two singlet states of CH2. Both the systems require a multireference formulation due to quasidegeneracy. In the field-free situation, the former encounters intruders at an intermediate trapezoidal geometry in the traditional treatment using effective Hamiltonians, while the latter shows a pronounced multireference character in the two singlet states. This affects the response properties in the presence of a perturbing field. A comparison with the full CI results in the same basis indicates the efficacy of the state-specific methods in wide ranges of geometries, even when the traditional effective Hamiltonian based methods fail due to intruders.
Two-dimensional vibrational spectroscopy. III. Theoretical description of the coherent two-dimensional IR-Raman spectroscopy for the investigation of the coupling between both IR- and Raman-active vibrational modes
Authors: Cho Minhaeng
Published: July 2002

Novel two-dimensional vibrational spectroscopic methods utilizing an infrared (IR) and two off-resonant optical pulses are theoretically studied. Unlike the coherent two-dimensional Raman or IR spectroscopies, the two successive vibrational coherence states are generated by an interaction with IR field as well as two interactions with off-resonant optical fields. Then, the emitted IR signal field is detected. The corresponding nonlinear response functions and their two-dimensional Fourier transforms are obtained in the analytical forms. It is shown that these methods are especially useful in studying the mode couplings and dynamics of only those vibrational modes that are both IR- and Raman active. A few numerically calculated 2D spectra are presented and compared with other types of coherent two-dimensional vibrational spectroscopies, such as coherent 2D Raman scattering or IR spectroscopies.
Phase space optimization of quantum representations: Direct-product basis sets
Authors: Poirier Bill , Light J. C.
Published: July 2002

The quantitative phase space similarities between the uniformly mixed ensembles of eigenstates, and the quasiclassical Thomas–Fermi distribution, are exploited in order to generate a nearly optimal basis representation for an arbitrary quantum system. An exact quantum optimization functional is provided, and the minimum of the corresponding quasiclassical functional is proposed as an excellent approximation in the limit of large basis size. In particular, we derive a stationarity condition for the quasiclassical solution under the constraint of strong separability. The corresponding quantum result is the phase space optimized direct-product basis—customized with respect to the Hamiltonian itself, as well as the maximum energy of interest. For numerical implementations, an iterative, self-consistent-field-like algorithm based on optimal separable basis theory is suggested, typically requiring only a few reduced-dimensional integrals of the potential. Results are obtained for a coupled oscillator system, and also for the 2D Henon–Heiles system. In the latter case, a phase space optimized discrete variable representation (DVR) is used to calculate energy eigenvalues. Errors are reduced by several orders of magnitude, in comparison with an optimized sinc-function DVR of comparable size.