bluuebunny/crossref_metadata_2025 huggingface dataset:
Globally uniform semiclassical wave functions for multidimensional systems
Authors: Madhusoodanan M. , Kay K. G.
Published: July 2002

The globally uniform semiclassical approximation for energy eigenstates developed by D. Zor and K. G. Kay [Phys. Rev. Lett. 76, 1990 (1996)] is derived explicitly for the case of multidimensional systems and is applied to two test cases. The adiabatic switching approximation is used to obtain various quantities that enter the semiclassical expression. Two formulations of the method are examined: one requiring several trajectories for each desired state and another requiring only a single trajectory per state. The multitrajectory version yields accurate results for all states investigated (overlap between semiclassical and quantum eigenstates >0.98), including some influenced by classical chaos. The single-trajectory treatment, however, is more efficient, gives accurate results for regular states, and is even applicable for certain chaotic states, although the multiple-trajectory method is preferred in such cases. Despite the substantial resemblance of the present theory to the frozen Gaussian approximation (FGA), it is a true semiclassical approximation and is found to produce wave functions that are significantly more accurate than those obtained from the FGA for all states examined.
A pair potentials study of matrix-isolated atomic zinc. I. Excited 1P1 state dynamics in solid Ar
Authors: Kerins P. N. , McCaffrey John G.
Published: July 2002

The pair-potentials calculations of McCaffrey and Kerins [J. Chem. Phys. 106, 7885 (1997)] used with success in simulating the emission spectroscopy of the Zn–RG matrix systems are extended to examine the different temporal decay characteristics exhibited at low temperature, T<13 K, by the singlet emission bands in the Zn–Ar matrix system. The 238 nm band, assigned in the earlier theoretical work to the body mode Q2, exhibits a 0.1 ns risetime, the 219 nm band assigned to the waist mode Q3, is prompt. By extracting the gradients and the second derivatives of the Q3 and Q2 mode potentials of a Zn⋅Ar18 cluster, decay rates of 3 and 2 ps, respectively, are calculated at the Franck–Condon regions of these potentials accessed in absorption, leading to effective competition between the Q2 and Q3 modes for relaxation of excited-state population and thereby to the coexistence of the 238 nm emission with the 219 nm band. A quasi-bound region is located at 0.32 Å in the body mode, Q2, which slows down the relaxation on this mode and is identified as responsible for the recorded risetime on the 238 nm emission. The temperature dependence exhibited in the Zn–Ar system at higher temperatures (T>14 K) in which the intensity of the 219 nm band can reversibly be put into the 238 nm band, was examined by generating the (PES) potential-energy surface for coupled Q2×Q3 vibronic modes. The theoretically predicted activation energy barrier is 380 cm−1, which is only in qualitative agreement with the value of 130.6 cm−1 extracted in the kinetics study. Possible reasons for the overestimation in the theoretical value are discussed.
Constant temperature molecular dynamics of a protein in water by high-order decomposition of the Liouville operator
Authors: Ishida Hisashi , Kidera Akinori
Published: July 2002

Among algorithms that are used to solve the equations of motion, the symplectic integrator (SI) has the advantage of conserving the phase space volume and ensuring a stable simulation. However, incorporating the explicit formula of the SI in a molecular simulation is feasible only for the systems whose Hamiltonian is described by K(p)+V(q), where the kinetic energy K and the potential energy V depend only on momenta p and coordinates q, respectively. Due to this limitation, explicit SI integrators cannot directly be applied to the Nosé-Hoover equations of motion for the constant temperature molecular dynamics (MD) simulation. In this article, by applying the formula of the decomposition of the exponential Liouville operator to the Nosé-Hoover equations, we have obtained a series of integrators for the constant temperature simulation which have the correct form of the Jacobian of the Nosé-Hoover equations. The systems examined here are liquid water and a protein in water. From the results of the constant temperature simulations, where several variations of the integrators were employed, we show that a combination of the Suzuki’s second order formula and the fourth order symplectic integrator of Calvo and Sanz-Serna generates a trajectory of much higher accuracy than the nonsymplectic Gear predictor-corrector method for a given amount of CPU time.
Second order Mo/ller–Plesset perturbation theory without basis set superposition error
Authors: Mayer I. , Valiron P.
Published: July 2002

A second order Mo/ller–Plesset perturbation theory which is free of the basis set superposition error (BSSE) is developed based on the “Chemical Hamiltonian Approach” (CHA). The zeroth order Hamiltonian is built up on the BSSE-free (but not orthogonal and not necessarily real) canonic CHA–SCF orbitals and their orbital energies. As the exclusion of BSSE makes the problem nonHermitian, biorthogonal perturbation theory is used to obtain the first order wave function. The second order energy is, however, calculated by using the conventional Hermitian Hamiltonian, in accord with the “CHA with conventional energy” recipe. For that reason we use a generalized Hylleraas functional introduced recently; this guarantees the second order energy to be real even in the case of complex CHA–SCF orbitals. The matrix elements entering the generalized Hylleraas functional are calculated by transforming all wave functions, creation and annihilation operators to an auxiliary orthonormalized basis. The new CHA-MP2 method has been tested on a number of van der Waals complexes and hydrogen bonded systems, by using a variety of different basis sets. In all cases a remarkable agreement has been found with the results given by the Boys and Bernardi’s counterpoise method (CP); this agreement is especially striking in the case of large and well-balanced basis sets. This indicates that the conceptually different CHA and CP schemes both take into account correctly the major BSSE effects.
Intermolecular interaction in the OH+–He and OH+–Ne open-shell ionic complexes: Infrared predissociation spectra of the ν1 and ν1+νb vibrations
Authors: Roth Doris , Nizkorodov Sergey A. , Maier John P. , Dopfer Otto
Published: July 2002

Midinfrared spectra of the OH+–He/Ne open-shell ionic complexes have been recorded by photofragmentation spectroscopy in a tandem mass spectrometer. The ν1 vibration (OH stretch) and its combination band with the intermolecular bending vibration (ν1+νb) have been observed for both complexes at the level of rotational resolution. The analysis of the spectra shows that both complexes have a linear proton-bound structure in their 3Σ− electronic ground states, with intermolecular center-of-mass separations of 2.60 and 2.65 Å for OH+–He and OH+–Ne, respectively. The ν1 vibrational origins are redshifted by 66.3 and 169.9 cm−1 with respect to the corresponding monomer transition indicating that the intermolecular interaction increases upon ν1 excitation. The fine structure of the ν1 (3Σ−←3Σ−) and ν1+νb (3Π←3Σ−) transitions arising from electron spin and vibrational angular momentum of νb have been analyzed in terms of a semirigid Hamiltonian including spin–spin, spin–rotation, and l-type doubling interaction terms. The molecular parameters extracted from the molecular constants provide valuable information on the radial and angular part of the intermolecular potential-energy surface in each considered vibrational state. The intermolecular interaction in OH+–Ne is stronger than in OH+–He, mainly due to the larger polarizability of the rare-gas atom.
The rotational spectrum of highly vibrationally mixed quantum states of propynol near 3330 cm−1
Authors: Green David , Holmberg Rebecca , Lee Chung Yi , McWhorter David A. , Pate Brooks H.
Published: July 2002

The rotational spectra of molecular eigenstates of propynol in the region of the acetylenic C–H stretch (3330 cm−1) have been measured using infrared-microwave saturation spectroscopy. These spectra illustrate the basic properties of the rotational spectra of highly vibrationally mixed quantum states. From the measurements we are able to measure the average value of the rotational constant and the width of the rotational constant distribution. We determine that the average value of the quantity 12 (B+C) is 17 MHz smaller than the ground state value (a decrease of 0.4%). The width of the distribution (FHWM) is 90 MHz (1% of the ground state value). The distribution is approximately Gaussian. Narrowing of the rotational spectrum of single eigenstates by intramolecular vibrational energy redistribution (IVR) exchange processes is observed for the Ka=2 eigenstates. From the spectral narrowing we determine that the average IVR lifetime for vibrational states with Ka=2 near 3330 cm−1 is approximately 75 ps, about five times faster than the IVR lifetime of the Ka=2 states following coherent vibrational excitation of the acetylenic C–H stretch bright state (400 ps). Weak narrowing of the Ka=0 and Ka=1 eigenstates is observed below J=2. We estimate the IVR lifetime for Ka=0 and Ka=1 states as approximately 600 ps. The strong Ka dependence of the IVR rates of the bath states indicates that strong parallel Coriolis interactions play a primary role in the energy redistribution process.
Nonresonant intermolecular spectroscopy beyond the Placzek approximation. II. Fifth-order spectroscopy
Authors: Murry Robert L. , Fourkas John T. , Keyes T.
Published: July 2002

We present an instantaneous normal mode analysis of the fifth-order polarizability-weighted densities of states for CS2 for experimentally relevant polarization conditions. We illustrate that polarization selectivity can be used to enhance selectively different contributions to the fifth-order spectrum. We show that the fifth-order spectrum arises predominantly from collision-induced scattering and from cross terms between collision-induced effects and librations, with only a vanishingly small pure librational contribution. We also demonstrate that there is no direct relationship between the third- and fifth-order Raman-weighted densities of states.
Reactive scattering of O− in organic films at subionization collision energies
Authors: Bass A. D. , Parenteau L. , Huels M. A. , Sanche L.
Published: July 2002

Anion desorption stimulated by the impact of 0–20 eV electrons on O2/hydrocarbon mixed films is reported. It is shown that part of the H−, OH−, CH−, and CH2− desorption yields from O2/hydrocarbon films is the result of reactive scattering of O− fragments produced via dissociative electron attachment (DEA) in the alkane and alkene thin films. These results support the interpretation that the DEA O− react with the hydrocarbon molecules to form a transient molecular anion complex which, in addition to autodetachment, may decay by dissociation into various anion and neutral fragments, and thus cause chemical modification of the solid.
Reactions of laser-ablated iron atoms with carbon monoxide: Infrared spectra and density functional calculations of FexCO, Fe(CO)x, and Fe(CO)x− (x=1,2,3) in solid argon
Authors: Zhou Mingfei , Chertihin George V. , Andrews Lester
Published: July 2002

Laser-ablated iron atoms have been reacted with CO molecules during condensation in excess argon. The FeCO molecule is observed at 1922.0 cm−1 in solid argon based on concentration studies, isotopic shifts, and density functional theory frequency calculations; the argon matrix redshifts this band 27.0 cm−1 (1.4%) from the high-resolution gas phase measurement. Absorptions at 1879.2 and 1984.8 cm−1 are assigned from isotopic substitution and density functional theory frequency calculations to the antisymmetric and symmetric vibrations of a bent Fe(CO)2 molecule in the matrix. The Fe(CO)x (x=3,4,5) molecules are also observed on annealing in agreement with earlier matrix work. Iron carbonyl ions were also produced and trapped: photosensitive absorptions at 1770.3, 1721.9, 1815.0, 1786.5, and 1853.5 cm−1 are assigned to FeCO−, Fe(CO)2−, Fe(CO)3−, and Fe(CO)4−, respectively, and a weak photosensitive 2081.5 cm−1 band is assigned to FeCO+. Polynuclear metal carbonyls were also formed on annealing; based on different CO concentration and laser power experiments and isotopic substitution, an 1897.7 cm−1 absorption is assigned to Fe2CO and a 1948.5 cm−1 band to Fe3CO.
An ab initio study of the Ar–HCN complex
Authors: Cybulski Sławomir M. , Couvillion Jacques , Kłos Jacek , Chałasiński Grzegorz
Published: July 2002

The potential energy surfaces for the ground state of the Ar–HCN complex have been calculated at several levels of theory, including the single and double excitation coupled-cluster method with noniterative perturbational treatment of triple excitation CCSD(T). Calculations have been performed using the augmented correlation-consistent polarized triple zeta basis set supplemented with bond functions (aug-cc-pVTZ+bf). The global minimum with a well depth of approximately 141 cm−1 has been found for the linear Ar–H–C–N geometry (Θ=0.0°) with the distance R between the Ar atom and the center of mass of the HCN molecule equal to 8.52a0. In addition, the potential energy surface has been found to contain a long channel that extended from the bent configuration at R=7.39a0 and Θ=59.7° (a well depth of 126 cm−1) toward the T-shaped configuration with R=7.16a0 and Θ=107.5° (a well depth of 121 cm−1). The interaction energies have been analyzed using perturbation theory of intermolecular forces. The location of the global minimum is determined by the anisotropy of the dispersion and induction effects. The ground vibrational state dissociation energy D0 determined by the collocation method has been found to be 105 cm−1. The wave number of the Σ1 bend amounts to 4.2 cm−1, somewhat below the experimental value (5.5 cm−1).
Spontaneous curvature-induced dynamical instability of Kirchhoff filaments: Application to DNA kink deformations
Authors: Haijun Zhou , Zhong-can Ou-Yang
Published: July 2002

The Kirchhoff elastic theory of thin filaments with spontaneous curvature is employed in the understanding of the onset of the kink transitions observed in short DNA rings. Dynamical analysis shows that when its actual curvature is less than some threshold value determined by the spontaneous curvature, a circular DNA will begin to buckle to other shapes. The observable and the dominant deformation modes are also determined by dynamical instability analysis, and the different effects of Zn2+ and Mg2+ ions on DNA configurational properties are qualitatively discussed.
A new interpretation of the OH-stretch spectrum of ice
Authors: Buch V. , Devlin J. P.
Published: July 2002

A new interpretation is presented of the vibrational band shapes of the proton disordered ice Ih and Ic in the OH and OD stretch region. The vibrationally excited states are shown to be related to those of a periodic system, whose unit cell is composed of four oscillating dipoles in a tetrahedral arrangement around an O-atom. Analysis of the excitations in this periodic system and of the perturbation induced by disorder is used to provide a detailed assignment of the band shapes of the measured spectra.
Pattern formation in unstable thin liquid films under the influence of antagonistic short- and long-range forces
Authors: Sharma Ashutosh , Khanna Rajesh
Published: July 2002

Various stages of evolution of the surface instability and pattern formation are investigated for unstable thin (<100 nm) fluid films subjected to the long-range van der Waals repulsion and a shorter range attraction. The complete three-dimensional morphology is resolved based on numerical solutions of the nonlinear 2D thin film equation. In the first phase of evolution, initial random nonhomogeneities are quickly reorganized into a small amplitude undulating structure consisting of long “hills” and “valleys.” Different types of patterns are formed thereafter, depending on the initial mean thickness vis-à-vis location of the minimum in the intermolecular force curve. Dewetting of relatively thick films occurs by circular isolated holes which grow and coalesce to form a large-scale structure with intervening pools and ridges of the liquid, which eventually decay into increasingly circular droplets. In thinner films, the shallow depressions merge and the long ridges of the bicontinuous structure mature, fragment, and directly transform into increasingly circular droplets, which continue to grow by ripening and merger. The characteristics of a pattern, its pathway of evolution, and the morphology at the onset of dewetting thus depend crucially on the form of the intermolecular potential in an extended neighborhood of the initial thickness. The linear and 1D nonlinear analyses used hitherto fail completely in prediction of morphological patterns, but can predict their length scales rather well.
The semiclassical regime of intramolecular vibrational dynamics
Authors: Gaspard P. , van Ede van der Pals P.
Published: July 2002

We report on a study of intramolecular dynamics after an ultrashort excitation. We show that a semiclassical regime of the vibrational dynamics exists on an intermediate time scale between the initial dephasing on the ultrashort time of the excitation pulse and the Heisenberg time scale associated with the mean level spacing. In this semiclassical regime, intramolecular dynamics presents time recurrences at the periods of the classical orbits. The amplitudes of these semiclassical time recurrences are evaluated for classically integrable and chaotic systems using Gutzwiller and the Berry–Tabor trace formulas.
The rotational and vibrational dynamics of argon–methane. I. A theoretical study
Authors: Heijmen Tino G. A. , Wormer Paul E. S. , van der Avoird Ad , Miller Roger E. , Moszynski Robert
Published: July 2002

Reported here is a theoretical calculation of the spectrum of the argon–methane complex based upon a previously reported ab initio potential energy surface [T. G. A. Heijmen et al., J. Chem. Phys. 107, 902 (1997)]. The irregular rotational structure observed in the spectrum is traced to a combination of the fact that the excited vibrational state of the methane has vibrational angular momentum and the methane rotates nearly freely within the complex. The theoretical spectrum is compared qualitatively with the experimental results obtained using the optothermal method, a more complete discussion of which is given in a companion paper, hereafter referred to as paper II.
Laser-induced fluorescence spectroscopy of the B̃ 2Π–X̃ 2A″ band system of HCCO and DCCO
Authors: Brock L. R. , Mischler B. , Rohlfing Eric A.
Published: July 2002

The electronic spectroscopy of the B̃ 2Π–X̃ 2A″ band system of the ketenyl radical, H(D)CCO, is investigated using laser-induced fluorescence (LIF) in a free-jet environment. Vibronically resolved excitation spectra for HCCO are obtained from 33 400 cm−1 to 35 100 cm−1; at higher energies the LIF spectra cutoff due to a rapid increase in the rate of predissociation. The parallel transitions from Ka″=1 in the ground state to the spin-orbit levels, 2Π3/2 and 2Π1/2, of the origin of the B̃ state are completely rotationally resolved for both HCCO and DCCO. Four other parallel transitions originating from Ka″=0 in the ground state and terminating on levels of Σ vibronic symmetry are observed and assigned to the two pairs of Σ states derived from one quantum of excitation in each of the Renner–Teller active modes, the CCO and CCH(D) bend. Rotational analysis provides effective rotational constants and spin-rotation (orbit) couplings for each of these levels. In addition to the Renner–Teller and spin-orbit couplings, there is substantial evidence for additional perturbations among the low-lying bending levels in the B̃ 2Π state of ketenyl.
The interaction of H2 with water ice by neutron scattering: Rotation and translation
Authors: Chen Zhan , Strauss H. L. , Loong C.-K.
Published: July 2002

The inelastic neutron scattering from H2 in D2O ice at energy transfers that correspond to rotation-translation are presented. Both the first energy gain and the energy loss peaks are observed. The position of the peaks shows that the H2 molecules execute nearly free rotation. The translational (recoil) contribution to the band positions as well as their widths can be fit by free translation with the molecule having an effective mass of about 1.7M0 where M0 is the mass of H2. The experimental results are compared to the available computer simulations.
Ab initio potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2
Authors: Cybulski Sl/awomir M. , Holt Jennifer S.
Published: July 2002

The three-dimensional ground state potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2 have been calculated using the single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)]. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions. Single point calculations for approximate minima have also been performed with several other basis sets including the quadruple zeta basis set (aug-cc-pVQZ) with bond functions. For He–Cl2 and Ar–Cl2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–Cl2 the CCSD(T) approach predicts the T-shaped configuration to be lower in energy. The linear configuration has been found to be more sensitive than the T-shaped one to the changes of the Cl–Cl bond length with the interaction becoming weaker when the Cl–Cl bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. The interaction in the T-shaped configuration becomes slightly stronger for shorter Cl–Cl bonds. For He–Cl2 and Ar–Cl2 the larger zero-point vibrational energy of the linear configuration is responsible for making the T-shaped configuration the ground vibrational state. Vibrational effects further increase the difference in energy between the ground state T-shaped configuration of Ne–Cl2 and its linear counterpart.
Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. III. Second order rotational angular momentum relaxation and Coriolis effects in Π←Σ bands
Authors: Hartmann J.-M. , Rodrigues R. , Nguyen-Van-Thanh  , Brodbeck C. , Boulet C. , Le Doucen R. , Lacome N. , Bonamy L.
Published: July 2002

The energy corrected sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Π←Σ bands of CO2–He and CO2–Ar mixtures in the 200–300 K temperature range. Measured line-broadening coefficients and absorption in the Q-branch of the ν2 band at moderate pressure are first used for the determination (from a fit) of the time constant associated with the relaxation of the second order traceless tensor of the rotational angular momentum (all other collisional quantities have been determined previously). The results obtained are consistent with previous (calculated) temperature dependent values of the depolarized Rayleigh cross sections. The model is then successfully tested through computations of absorption in the ν2 and (ν1+ν2)I bands at elevated densities. Analysis of line-mixing effects is made, including study of the influence of interbranch transfers and of Coriolis coupling. Differences between the effects of collisions with He and Ar are pointed out and explained.
Semiclassical molecular dynamics simulations of excited state double-proton transfer in 7-azaindole dimers
Authors: Guallar Victor , Batista Victor S. , Miller William H.
Published: July 2002

An ab initio excited state potential energy surface is constructed for describing excited state double proton transfer in the tautomerization reaction of photo-excited 7-azaindole dimers, and the ultrafast dynamics is simulated using the semiclassical (SC) initial value representation (IVR). The potential energy surface, determined in a reduced dimensionality, is obtained at the CIS level of quantum chemistry, and an approximate version of the SC-IVR approach is introduced which scales linearly with the number of degrees of freedom of the molecular system. The accuracy of this approximate SC-IVR approach is verified by comparing our semiclassical results with full quantum mechanical calculations. We find that proton transfer usually occurs during the first intermonomer symmetric-stretch vibration, about 100 fs after photoexcitation of the system, and produces an initial 15 percent population decay of the reactant base-pair, which is significantly reduced by isotopic substitution.
Ultraviolet photodissociation of furan probed by tunable synchrotron radiation
Authors: Sorkhabi Osman , Qi Fei , Rizvi Abbas H. , Suits Arthur G.
Published: July 2002

The photodissociation dynamics of furan at 193 nm have been studied using photofragment translational spectroscopy with tunable vacuum ultraviolet (VUV) probe provided by synchrotron radiation on the Chemical Dynamics Beamline at the Advanced Light Source. Three primary channels are observed: HCO+C3H3, CO+C3H4, and H2CCO+C2H2. The evidence suggests that the two closed-shell channels occur on the ground-state potential energy surface (PES) following internal conversion, while the radical channel likely takes place on an excited PES. All channels exhibit a barrier for dissociation with the acetylene+ketene channel having the largest value at about 25 kcal/mol. Angular distribution measurements show anisotropy only for the radical channel. These findings are consistent with a rapid excited state dissociation for the radical channel and slow dissociation for the other two pathways. The two ground-state dissociation channels—propyne+CO and acetylene+ketene—should be important in the thermal decomposition of furan as was found in pyrolytic studies [A. Lifshitz, M. Bidani, and S. Bidani, J. Phys. Chem. 90, 5373 (1986)] and theoretical investigations [R. Liu, X. Zhou, and L. Zhai, J. Comput. Chem. 19, 240 (1998)].
Structural information from two-dimensional fifth-order Raman spectroscopy
Authors: Okumura Ko , Tokmakoff Andrei , Tanimura Yoshitaka
Published: July 2002

Two-dimensional (2D) fifth-order Raman spectroscopy is a coherent spectroscopy that can be used as a structural tool, in a manner analogous to 2D nuclear magnetic resonance (NMR) but with much faster time scale. By including the effect of dipole-induced dipole interactions in the molecular polarizability, it is shown that 2D Raman experiments can be used to extract distances between coupled dipoles, and thus elucidate structural information on a molecular level. The amplitude of cross peaks in the 2D Raman spectrum arising from dipole-induced dipole interactions is related to the distance between the two dipoles (r) and the relative orientation of the dipoles. In an isotropic sample with randomly distributed dipole orientations, such as a liquid, the cross peak amplitude scales as r−6. In an anisotropic sample such as a solid, where the orientational averaging effects do not nullify the leading order contribution, the amplitude scales as r−3. These scaling relationships have analogy to the dipole coupling relationships that are observed in solid state and liquid 2D NMR measurements.
New methods for incorporating nonholonomic constraints into molecular dynamics simulations
Authors: Kutteh Ramzi
Published: July 2002

Three new algorithms are presented for incorporating nonholonomic constraints into molecular dynamics (MD) simulations, along with any additional holonomic constraints. The advantages of these algorithms over the commonly used Gaussian approach are discussed. Of the three algorithms presented, the optimal one can efficiently ensure satisfaction of large numbers of nonholonomic and holonomic constraints at every MD time step, without introducing additional numerical errors in the coordinate or velocity trajectories. Numerical results from MD simulations of Lennard-Jones particles, rigid water molecules, and partially rigid methane molecules are given, illustrating the advantages of this algorithm. In addition, this algorithm is suggested as a more advantageous alternative to velocity scaling, for maintaining fixed temperature during equilibration of constant energy MD simulations.
Classical mechanics and the spreading of localized wave packets in condensed phase molecular systems
Authors: Herman Michael F. , Coker David F.
Published: July 2002

The relationship between the diverging of classical trajectories in chaotic many-body systems, the spreading of quantum wave packets, and the validity and use of classical molecular dynamics is explored. This analysis, which is based on the semiclassical description of wave function propagation in terms of a weighted integration over a traveling fixed width coherent state basis, suggests that the exponential divergence of nearby classical trajectories in chaotic many-body systems should result in the rapid delocalization of an initially localized quantum wave packet describing the state of the system. Thus the justification for the use of classical molecular dynamics procedures for these supposedly classical systems cannot be based on the picture of the system wave function remaining localized as its center follows a nearly classical trajectory. The quantum evolution of the system density, on the other hand, requires two propagators, and each of these propagators is represented as an integration over trajectories in the semiclassical picture. The interference between the contributions from these two integrations over classical trajectories focuses the analysis on the most important points in this trajectory pair space, which are shown to occur when both trajectories in the pair are the same. Given reasonable assumptions for the initial density for a system that is expected to be well described by classical molecular dynamics, and given an appropriate choice for the width of the coherent state basis which is employed in the semiclassical description, it is shown that the semiclassical expressions for time dependent observables and correlation functions reduce the purely classical expressions, despite the fact that an initially localized wave packet would rapidly delocalize for the same system.
Correlated electron extracule densities in position and momentum spaces
Authors: Gálvez F. J. , Buendı́a E. , Sarsa A.
Published: July 2002

Spherically averaged extracule densities in position, d(R), and momentum, d̄(P), spaces have been calculated for the atoms helium to neon starting from explicitly correlated wave functions. Correlated values for the electron–electron counterbalance density in position, d(0), and in momentum, d̄(0), spaces, and also for the expectation values 〈Rn〉 and 〈Pn〉 are reported. A systematic study of the electronic correlation has been performed by comparing the correlated results with the corresponding Hartree–Fock ones.
Adiabatic states of ozone using Fock space multireference coupled cluster method
Authors: Vaval Nayana , Pal Sourav
Published: July 2002

In this paper we present the Fock space multireference coupled cluster theory suitable for calculation of low-lying adiabatically excited or electron attached states. Low-lying adiabatic as well as vertical excited states of ozone are calculated using this theory in singles and doubles approximation. The calculated adiabatic excitation energies are compared with the experimental values. We also report the adiabatic electron affinity value of ozone.
Electron-pair densities and their moments of heavy atoms Cs through Lr
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

Spherically averaged electron-pair intracule (relative motion) h(u) and extracule (center-of-mass motion) d(R) densities are constructed for the 49 heavy atoms Cs (atomic number Z=55) through Lr (Z=103), by using the numerical Hartree–Fock method. As known for the lighter atoms from He (Z=2) to Xe (Z=54), all the electron-pair densities h(u) and d(R) of the heavy atoms are found to be monotonically decreasing functions even when f subshells are occupied. The intracule 〈un〉 and extracule 〈Rn〉 moments and the electron–electron coalescence h(0) and counterbalance d(0) densities are also obtained, and their dependence on Z is discussed. The present results for Z=55–103, together with the previous ones [J. Chem. Phys. 107, 8510 (1997); 108, 3424 (1998)] for Z=2–54, provide a compilation of accurate Hartree–Fock values of the electron-pair properties for the 102 neutral atoms in the periodic table.
An ab initio study of the ground states and some excited states of BeRG, Be+RG, and Be+2RG van der Waals complexes (RG=He, Ne)
Authors: Leung Allen W. K. , Breckenridge W. H.
Published: July 2002

By ab initio methods, we have characterized the potential curves of the unusual, doubly excited valence van der Waals states, Be(2pπ+12pπ−1 3PJ)⋅RG[3Σ−], where RG=He, Ne. Similar to the Mg(3pπ+13pπ−1 3PJ)⋅RG[3Σ−] states (RG=He, Ne, Ar, Kr) which were characterized experimentally and theoretically earlier, these Be(2pπ2pπ 3PJ)⋅RG[3Σ−] states are much more strongly bound than their singly excited Be(2s2pπ 3PJ)⋅RG[3Π] analogs, and even much more strongly bound than the analogous Be+(2s 2S)⋅RG[2Σ+] ground-state ions. This is attributed to the lack of a large Be(2s) electron cloud with density along the internuclear axis, so that quadrupole/induced-dipole and dispersion attraction forces can proceed to much smaller internuclear distances before repulsion sets in. The BeHe[3Σ−] state is also almost five times more bound than the BeNe[3Σ−] state, despite the fact that the polarizability of the He atom is only one-half that of the Ne atom. This is again attributed to minimization of repulsive forces, since strong Be(2pπ)/RG(npπ) exchange repulsion is completely absent when RG=He. The fact that the bond strengths of the Be(2pπ2pπ)⋅He[3Σ−] and Be+(2pπ)⋅He[2Π] states are quite similar, and only about 20% less than the bond strength of the free Be++⋅He[1Σ+] ion, is consistent with this interpretation. All of the strongly bound Be⋅RG neutral and ionic states calculated here have bond strengths greater than, and bond lengths smaller than, their Mg⋅RG analogs. This is because the Be electron clouds are all smaller than their Mg analogs, so that, again, all attractive forces can proceed to smaller distances before repulsion sets in.
Voronoi polyhedra analysis of the local structure of water from ambient to supercritical conditions
Authors: Jedlovszky Pál
Published: July 2002

The changes of the local structure of water are investigated with increasing temperature and decreasing density from ambient conditions up to the critical point. The local structure around the molecules is analyzed in terms of the properties of the Voronoi polyhedra (VP) of the molecules. For reference, the entire analysis is also performed in liquid H2S. In this way, the effect of the hydrogen bonding on the local structure can also be studied. In determining the VP of the molecules an exact algorithm is used. In analyzing the local structure it is found that the decrease of the density not only leads to the increase of the free volume available for the molecules, but it also increases the volume of the voids present in the system. A linear relation is found between the average free volume of the molecules and the average volume of the vacancies, the latter being always about the double of the former. The comparison of the results concerning the shape of the VP of the molecules obtained for water at different thermodynamic state points and for liquid H2S clearly reveals that even above the critical point, hydrogen bonds are still playing a very important role in forming the molecular level structure of water. It is also found that the tetrahedral arrangement of the nearest-neighbor molecules, which is a characteristic feature of the water structure under ambient conditions, already disappears at temperatures well below the critical point, and the relative arrangement of the first four nearest-neighbor molecules becomes rather similar to that in liquid H2S.
Growth of nanometer thin ice films from water vapor studied using scanning polarization force microscopy
Authors: Bluhm Hendrik , Salmeron Miquel
Published: July 2002

Atomic force microscopy (AFM) was used to study the growth and morphology of ice films on the cleavage surface of mica. Measurements performed in contact, as well as in noncontact operation modes of the microscope, allowed us to distinguish the solid and liquid parts of the film. At temperatures below −30 °C, supercooled water droplets formed on top of a thin (nanometer range) ice layer in contact with the substrate. After annealing, a contiguous flat film was formed. Between −20 and −10 °C and at a relative humidity of ∼83%, the film consisted of a solid ice layer ∼7 Å thick, covered by a liquid-like layer 50±5 Å thick. When the temperature was raised above 0 °C, droplets formed, which subsequently evaporated. Comparison of results obtained in the various AFM operation modes allowed us to conclude the existence of a liquid-like layer on the ice surface.
Lattice model of living polymerization. I. Basic thermodynamic properties
Authors: Dudowicz Jacek , Freed Karl F. , Douglas Jack F.
Published: July 2002

A Flory-Huggins type lattice model of living polymerization is formulated, incorporating chain stiffness, variable initiator concentration r, and a polymer-solvent interaction χ. Basic equilibrium properties [average chain length L, average fraction of associated monomers Φ, specific heat CP, entropy S, polymerization temperature Tp, and the chain length distribution p(N)] are calculated within mean-field theory. Our illustrative calculations are restricted to systems that polymerize upon cooling [e.g., poly(α-methylstyrene)], but the formalism also applies to polymerization upon heating (e.g., sulfur, actin). Emphasis is given to living polymer solutions having a finite r in order to compare theory with recent experiments by Greer and co-workers, whereas previous studies primarily focused on the r→0+ limit where the polymerization transition has been described as a second order phase transition. We find qualitative changes in the properties of living polymer solutions for nonzero r: (1) L becomes independent of initial monomer composition φm0 and temperature T at low temperatures [L(T≪Tp)∼2/r], instead of growing without bound; (2) the exponent describing the dependence of L on φm0 changes by a factor of 2 from the r→0+ value at higher temperatures (T⩾Tp); (3) the order parametertype variable Φ develops a long tail with an inflection point at Tp; (4) the specific heat maximum CP* at Tp becomes significantly diminished and the temperature range of the polymer transition becomes broad even for small r [r∼O(10−3)]. Moreover, there are three characteristic temperatures for r>0 rather than one for r→0: a “crossover temperature” Tx demarking the onset of polymerization, an r-dependent polymerization temperature Tp defined by the maximum in CP (or equivalently, the inflection point of Φ), and a “saturation temperature” Ts at which the entropy S of the living polymer solution saturates to a low temperature value as in glass-forming liquids. A measure of the “strength” of the polymerization transition is introduced to quantify the “rounding” of the phase transition due to nonzero r. Many properties of living polymer solutions should be generally representative of associating polymer systems (thermally reversible gels, colloidal gels, micelles), and we compare our results to other systems that self-assemble at equilibrium.
Experimental and theoretical study of line mixing in methane spectra. II. Influence of the collision partner (He and Ar) in the v3 IR band
Authors: Pieroni D. , Nguyen-Van-Thanh  , Brodbeck C. , Hartmann J.-M. , Gabard T. , Champion J.-P. , Bermejo D. , Domenech J.-L. , Claveau C. , Valentin A. , Tonkov M. V. , Grigoriev I. M. , Le Doucen R.
Published: July 2002

Line mixing effects are studied in the v3 band of CH4 perturbed by Ar and He at room temperature. Experiments have been made in the 2800–3200 cm−1 spectral region using four different setups. They cover a wide range of total densities, including low (0.25–2 atm), medium (25–100 atm), and high (200–1000 atm) pressure conditions. Analysis of the spectra demonstrates that the spectral shapes (of the band, the Q branch, the P and R manifolds,…) are significantly influenced by line mixing. The theoretical approach proposed in the preceding paper is used in order to model and analyze these effects. As done previously, semiclassical state-to-state rates are used together with a few empirical constants. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They prove the quality of the approach which satisfactorily accounts for the effects of pressure and of rotational quantum numbers on the spectral shape. It is shown that collisions with He and Ar lead to different line-coupling schemes (e.g., more coupling within the branches and less between branches) and hence to different shapes. The influence of line coupling between different branches and manifolds is evidenced and studied using high pressure spectra and absorption in the band wings.
A unified kinetic approach to binary nucleation
Authors: Kevrekidis P. G. , Lazaridis M. , Drossinos Y. , Georgopoulos P. G.
Published: July 2002

Two different methods to calculate the steady-state nucleation rate in heteromolecular systems proposed by Stauffer (1976) and Langer (1969) are analyzed. Their mathematical equivalence is explicitly demonstrated, thereby obtaining a generic expression for the rate of binary nucleation. Its numerical evaluation does not entail rotation of the coordinate system at the saddle point, but it only requires data in the natural coordinate system of number fluctuations, namely molecular impingement rates, the droplet free energy and its second order derivatives at the saddle point, and the total density of condensible vapors.
Variation of the linewidth of the A0+←X 1Σ+ transition of NaI
Authors: Baba Masaaki , Kokita Takao , Kasahara Shunji , Katô Hajime
Published: July 2002

A continuous wave (cw) tunable single mode laser beam was propagated perpendicular to a collimated effusive molecular beam of NaI, and the excitation spectra of the A0+(v′,J′)−X 1Σ+(v″,J″) transitions of the NaI molecule were measured by monitoring the intensity of the molecular fluorescence. A large variation of the linewidth and line intensity with the rotational quantum number J was observed. The linewidths of rotationally resolved lines, attributable to lifetime broadening originating from predissociation, were obtained by a fitting of the observed spectral line shape to a Voigt profile.
Photoionization-efficiency spectrum and ionization energy of C2H5SCl
Authors: Cheng Bing-Ming , Chew Eh Piew , Liu Chin-Ping , Yu Jen-Shiang K. , Yu Chin-hui
Published: July 2002

Photoionization-efficiency (PIE) spectra in the wavelength range of 108–143 nm are measured for C2H5SCl produced from reaction systems Cl/Cl2/C2H5SH and Cl/Cl2/C2H5SSC2H5 in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as the source of radiation. According to PIE spectra of C2H5SCl thus obtained, the ionization energy (IE) is (8.994±0.007) eV. Based on GAUSSIAN-2 calculations, the observed ionization of C2H5SCl near the threshold region is likely to form doublet C2H5SCl+ from singlet C2H5SCl; the calculated IE 8.978 eV agrees well with the experimental value. A vibrational frequency of doublet C2H5SCl+, was found to be (557±60) cm−1, which agrees satisfactorily with a theoretical value of 560.9 cm−1.
An efficient Brownian dynamics method for calculating the time-dependent rate coefficients of diffusion-influenced reactions
Authors: Yang Seongeun , Kim Joohyun , Lee Sangyoub
Published: July 2002

The Brownian dynamics (BD) method for diffusion-influenced bimolecular reactions, which was proposed by Lee and Karplus, is implemented to calculate the time-dependent rate coefficient kf(t). It is shown that the present method gives quick estimates of kf(t) by using just about 5000 trajectories. Compared to the previous method due to Zhou, which has a similar efficiency and accuracy, the present method has a key advantage. It requires only one set of BD simulations to calculate the time-dependent rate coefficients for any value of intrinsic rate constant, while Zhou’s method requires separate sets of BD simulations for different intrinsic rate constants.
Localized-density-matrix, segment-molecular-orbitals and poly(p-phenylenevinylene) aggregates
Authors: Yokojima Satoshi , Wang XiuJun , Zhou DongHao , Chen GuanHua
Published: July 2002

The segment-molecular-orbital representation is developed and incorporated into the recently developed linear-scaling localized-density-matrix method. The entire system is divided into many segments, and the molecular orbitals of all segments form the basis functions of the segment-molecular-orbital representation. Introduction of different cutoff lengths for different segment-molecular-orbitals leads to a drastic reduction of the computational cost. As a result, the modified localized-density-matrix method is employed to investigate the optical responses of large Poly(p-phenylenevinylene) aggregates. In particular, the interchain excitations are studied. The complete neglect of differential overlap in spectroscopy hamiltonian is employed in the calculation.
Study of the ultrasonic relaxation and cis–trans isomerization in propyl formate: Evidence from low frequency spectrometry
Authors: Mirzaev S. Z. , Khabibullaev P. K. , Kononenko V. S. , Saidov A. A.
Published: July 2002

The ultrasonic attenuation of pure propyl formate was measured in the frequency range from 80 kHz to 150 MHz, over the temperature range from 20 °C to 70 °C. From the temperature dependence of a relaxation parameter of the ultrasonic attenuation, molecular parameters of propyl formate were calculated. The estimated value were 5.5 kcal/mol for activation enthalpy and 3.2 kcal/mol for the enthalpy difference between trans- and cis-isomers of propyl formate.
Scattering of xenon from Ni(111): Collision-induced corrugation and energy transfer dynamics
Authors: Ellison Mark D. , Matthews Carl M. , Zare Richard N.
Published: July 2002

Experiments have been conducted in which a beam of xenon atoms collides with a clean Ni(111) surface, and the speed and angular distributions of the scattered Xe atoms are measured for different incident energies, incident angles, and surface temperatures. At high incident energies, the translational energy of the scattered Xe is independent of initial and final scattering angles. This result is attributed to multiple xenon-surface collisions prior to Xe escape. At lower incident energies, the scattering behavior depends more on the scattering angle. Interestingly, a small fraction of Xe is trapped on a 250 K Ni(111) surface at high incident translational energies.
Self-consistent harmonic theory of solvation in glassy systems: Classical solvation
Authors: Reichman David R. , Voth Gregory A.
Published: July 2002

Various harmonic theories of classical solvation dynamics in glassy systems are discussed. The “optimized normal mode” theory is found to provide a substantial improvement over more standard normal mode approaches for the description of solvation dynamics in both glassy and supercooled media. A methodology is developed to include all multiphonon terms in the expansion of the collective solvation coordinate, thus going beyond “linear” solvation theories. The results suggest that the methods described here can provide a quantitative description of solvation over a wide temperature range in systems of low diffusiveness. Lastly, the extension of Zwanzig’s model of self-diffusion in supercooled media to the treatment of solvation phenomena is discussed.