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The calculation of two-electron repulsion integrals (ERIs) is a crucial aspect of Hartree–Fock calculations. In computing the ERIs of varying angular momentum, both the central processing unit (CPU) and the graphics processing unit (GPU) have their respective advantages. To accelerate the ERI evaluation and Fock matrix generation, a hybrid CPU/GPU method has been proposed to maximize the computational power of both CPU and GPU while overlapping the CPU and GPU computations. This method employs a task queue where each task corresponds to ERIs with the same angular momentum. The queue begins with ERIs of low angular momentum, which are computationally efficient on GPUs, and ends with ERIs of high angular momentum, which are better suited for CPU computation. CPUs and GPUs dynamically grab and complete tasks from the start and end of the queue using OpenMP dynamic scheduling until all tasks are finished. The hybrid CPU/GPU computation offers the advantage of enabling calculations with arbitrary angular momentum. Test calculations showed that the hybrid CPU/GPU algorithm is more efficient than "GPU-only" when using a single GPU. However, as more GPUs are involved, the advantage diminishes or disappears. The scaling exponents of the hybrid method were slightly higher than "GPU-only," but the pre-exponent factor was significantly lower, making the hybrid method more effective overall. [ABSTRACT FROM AUTHOR]

In this work we report an ab initio intermolecular potential energy surface and theoretical spectroscopic studies for Xe–H2O complex. The ab initio energies are calculated with CCSD(T) method and large basis sets (aug-cc-pVQZ for H and O and aug-cc-pVQZ-PP for Xe) augmented by a {3s3p2d2f1g} set of bond functions. This potential energy surface has a global minimum corresponding to a planar and nearly linear hydrogen bonded configuration with a well depth of 192.5 cm-1 at intermolecular distance of 4.0 Å, which is consistent with the previous determined potential by Wen and Jäger [J. Phys. Chem. A 110, 7560 (2006)]. The bound state calculations have been performed for the complex by approximating the water molecule as a rigid rotor. The theoretical rotational transition frequencies, isotopic shifts, nuclear quadrupole coupling constants, and structure parameters are in good agreement with the experimental observed values. The wavefunctions are analyzed to understand the dynamics of the ground and the first excited states. [ABSTRACT FROM AUTHOR]

The initial state selected time-dependent wave packet method has been extended to calculate the total reaction probability for atom-triatom reactions with total angular momentum J > 0 by treating both bonds in the triatom reagent reactively. The total exchange and abstraction reaction probabilities for the title reaction with J = 15 calculated with 2 K-blocks (the projection of the total angular momentum on the body-fixed axis) show that one has to treat both OH bonds in the H[SUB2]O reagent reactively for the exchange reaction, but for the abstraction reaction one can treat one OH bond as a spectator bond to get accurate results. This is in accord with what had been found for the total reaction probabilities for J=0 [Phys. Rev. Lett. 89, 103201 (2002)]. The J= 15 reaction probabilities also show that the CS (centrifugal sudden) approximation is inadequate for the title reaction, in particular for the abstraction reaction. The integral cross sections for the abstraction reaction, calculated without the CS approximation but by freezing one OH bond in the H[SUB2]O reagent for the ground and first vibrationally excited states, are found to be substantially larger than the corresponding CS integral cross sections. However, the integral cross section for the ground vibrational state is still much smaller than the existing experimental result. Since the treatment of freezing OH bond is not expected to introduce any noticeable error, it is conceivable that the experiments overestimated the integral cross section for the abstraction reaction. [ABSTRACT FROM AUTHOR]

The initial state selected time-dependent wave packet method has been developed to study the H+ CH[sub 4] reaction in seven dimensions by employing the model developed by Palma and Clary [J. Chem. Phys. 112, 1859 (2000)]. The latter eight-dimensional model only assumes that the nonreacting CH[sub 3] group keeps a C[sub 3V] symmetry in reaction. The CH bond lengths in the nonreacting CH[sub 3] group were fixed in the study to reduce the number of degrees of freedom to seven. Our calculations reveal that it is very important to accurately deal with the umbrella motion of the CH[sub 3] group while studying this reaction. We investigated the effects of the fundamental vibrational excitations of CH[sub 4] on the reaction. Finally, we compare our rate constant for the ground rovibrational initial state with available experimental and other theoretical results. [ABSTRACT FROM AUTHOR]

This paper presents fully converged integral cross sections for the D[sub 2](j[sub 1]=0,2,4)+OH(j[sub 2]=0,2,4)→HOD+D reaction on the YZCL2 potential energy surface. It is found that the influence of reagent rotation excitation on reactivity is relatively mild compared with that of the D[sub 2] vibration excitation, but is fairly important to the thermal rate constant at low temperature. In general, the OH rotation excitation tends to hinder the reaction, and the D[sub 2] rotation excitation tends to enhance the reactivity. The simultaneous reagent rotation has some correlated effect, with the influence of the D[sub 2] rotation excitation considerably dependent on the OH rotation excitation. It is also found that the reaction is strongly dependent on j[sub 12] (the coupled angular momentum between j[sub 1] and j[sub 2]), K[sub 0] (the projection of the reagent rotation on the body fixed axis), and the parity of the system, in particular for either the OH or D[sub 2] rotationally excited initial states, but this dependence becomes relatively weaker as the collision energy increases. Excellent agreement achieved between theory and experiment on the thermal rate coefficients for temperatures up to 400 K clearly shows that the YZCL2 potential energy surface is capable of producing “quantitatively” accurate results for the title reaction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report dynamics on two interpolated global potential energy surfaces (PES) for the H[sub 2]+OH reaction. The first PES is based on both QCISD(T)/6-311++G(3df,2pd) and MRCI/aug-cc-pVTZ ab initio calculations. In the second version, the energies at the interpolation data points are improved to the UCCSD(T)/aug-cc-pVQZ level. Fully converged integral cross sections were calculated on these two PESs for the title reaction for the ground rovibrational and two rotationally excited initial states. Good agreement between theory and experiment for rate coefficients for temperatures up to 1050 K are only achieved on the second PES. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper, we performed a theoretical study on the influence of intramolecular bond lengths on the intermolecular interactions between H2 and AgCl molecules. Using four sets of bond lengths for the monomers of H2 and AgCl, four-dimensional intermolecular potential energy surfaces (PESs) were constructed from ab initio data points at the level of single and double excitation coupled cluster method with noniterative perturbation treatment of triple excitations. A T-shaped global minimum was found on the PES. Interestingly, both the binding energies and Ag–H2 distances present a linear relationship with the intramolecular bond lengths of H2–AgCl. The accuracy of these PESs was validated by the available spectroscopic data via the bound state calculations, and the predicted rotational transition frequencies can reproduce the experimental observations with a root-mean-squared error of 0.0003 cm−1 based on the PES constructed with r(H–H) and r(Ag–Cl) fixed at 0.795 and 2.261 Å, respectively. The intermolecular vibrational modes were assigned unambiguously with a simple pattern by analyzing the wave functions. Isotope effects were also investigated by the theoretical calculations, and the results are in excellent agreement with the available spectroscopic data. The transition frequencies for the isotopolog D2–AgCl are predicted with the accuracy of 0.3 MHz. [ABSTRACT FROM AUTHOR]

The mode selectivity in the prototypical H + CH3D reaction is investigated by the initial state selected time-dependent wave packet method within a ten-dimensional quantum dynamics model. The model is a novel reduced dimensional model for the X + YCZ3 reaction, which allows the CZ3 to break C3V symmetry. The calculated reaction probabilities initially from different reactant vibrational states show that the CH3 stretching modes excitations obviously promote the H-abstraction reaction but have a slight influence on the D-abstraction reaction. In contrast, the CD stretching mode excitation significantly enhances the D-abstraction reaction. For both H- and D-abstraction reactions, the excitation of either the CH3 umbrella bending mode or the CH3 rocking mode shows a promotional effect on the reactivity, while fundamental excitation of the CH3 bending mode has a negligible effect. Impressively, the first-overtone excitation of CH3 bending mode remarkably promotes the H-abstraction reaction, resulting from the 1:2 Fermi coupling between the CH3 symmetric stretching mode and the first overtone of CH3 bending mode. In addition, translational energy is more efficient than vibrational energy in promoting the H-abstraction reaction at low energy, while vibrational energy becomes more efficient for the D-abstraction reaction. [ABSTRACT FROM AUTHOR]

Accurate global ab initio surfaces are presented for the title reactions and competing exchange processes. The ab initio calculations were based on the quadratic configuration interaction treatment with all single and double excitations and perturbative account for triple excitations together with a 6-311++G(3df,2pd) basis, the multi-reference configuration interaction method with the Davidson correction procedure together with an aug-cc-pVTZ basis, and the unrestricted coupled-cluster treatment with all single and double excitations and perturbative account for triple excitations together with an aug-cc-pVQZ basis. The surfaces are interpolations of ab initio data for energies, energy gradients, and second derivatives at configurations scattered throughout the relevant configuration space. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A full-dimensional quantum dynamical study is reported for the title reaction on the YZCL2 potential energy surface. The influence of reagent rotation on the dynamics is examined in detail. It is found that the rotational excitation of the HD reagent substantially enhances the rate coefficient for forming the H[sub 2]O+D product, and plays an important role in determining the branching ratio at low temperatures. The theoretical results are compared with available experimental data for an integral cross section, thermal rate coefficients, and their corresponding branching ratios. Excellent agreement between theory and experiment is revealed for available thermal rate coefficients and its branching ratio. The agreement between theory and experiment for the integral cross section is also satisfactory. The study clearly shows that the YZCL2 potential energy surface is capable of producing "quantitatively" accurate results for the title reaction. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This work presented a new model, Polarizable Fragment Density Model (PFDM), for the fast energy estimation of peptides, proteins, or other large molecular systems. By introducing an analogous relation to the virial theorem, the kinetic energy in Kohn–Sham Density Functional Theory (DFT) is approximated to the corresponding potential energy multiplied by a scale factor. Furthermore, the error due to this approximation together with the exchange–correlation energy is approximated as a second order Taylor's expansion about density. The PFDM energy is expressed as a functional of electronic density with system-dependent model parameters, such as a scaling factor c and a series of atomic pairwise KAB. The electron density in PFDM consists of a frozen part retaining chemical bonding information and a polarizable part to describe polarization effects, both of which are expanded as a linear expansion of Gaussian basis functions. The frozen density can be pre-calculated by fitting the DFT calculated density of fragments, as well as the polarizable density is optimized to solve PFDM energy. The PFDM energy is a quadratic function of the expansion coefficients of polarizable density and can be solved without expensive iteration process and numerical integrals. PFDM is especially suitable for the energy calculation of large molecular system with identical subunits, such as proteins, nucleic acids, and molecular clusters. Applying the PFDM method to the proteins, the results show that the accuracy is comparable to the PM6 semi-empirical method, and the efficiency is one order of magnitude faster than PM6. [ABSTRACT FROM AUTHOR]

The reaction dynamics of hydroxyl radical with methane has been investigated using time-dependent wave packet approach within reduced six- and seven-dimensional models. Initial state-selected total reaction probabilities and integral cross sections for the hydrogen abstraction reaction have been computed on the empirical potential energy surface developed by Espinosa-García et al. [J. Chem. Phys. 112, 5731 (2000)]. Excitations of the CH stretching mode and/or the CH3 umbrella mode enhance the reaction. They are, however, both less efficient than translational energy in promoting the reaction, at least at low collision energies. Also, we studied the accuracy of two approximations: centrifugal sudden (CS) and J-shifting (JS), in the calculations of the integral cross sections by a comparison to coupled-channel (CC) calculations. The integral cross sections obtained indicated that the CS approximation works well over the whole energy range studied, and the JS approximation gives accurate cross sections at low collision energies, while noticeably overestimates them at relatively high collision energies. In addition, the OH radical acts as a good spectator as it has a negligible effect on the reaction. [ABSTRACT FROM AUTHOR]

Full-dimensional quantum calculations of the vibrational states of H5+ have been performed on the accurate potential energy surface developed by Xie et al. [J. Chem. Phys. 122, 224307 (2005)]. The zero point energies of H5+, H4D+, D4H+, and D5+ and their ground-state geometries are presented and compared with earlier theoretical results. The first 10 low-lying excited states of H5+ are assigned to the fundamental, overtone, and combination of the H2-H3+ stretch, the shared proton hopping and the out-of-plane torsion. The ground-state torsional tunneling splitting, the fundamental of the photon hopping mode and the first overtone of the torsion mode are 87.3 cm-1, 354.4 cm-1, and 444.0 cm-1, respectively. All of these values agree well with the diffusion Monte Carlo and multi-configuration time-dependent Hartree results where available. [ABSTRACT FROM AUTHOR]

On a recent analytical potential energy surface developed by two of the authors, an exhaustive kinetics study, using variational transition state theory with multidimensional tunneling effect, and dynamics study, using both quasi-classical trajectory and full-dimensional quantum scattering methods, was carried out to understand the reactivity of the NH3 + H → NH2 + H2 gas-phase reaction. Initial state-selected time-dependent wave packet calculations using a full-dimensional model were performed, where the total reaction probabilities were calculated for the initial ground vibrational state and for four excited vibrational states of ammonia. Thermal rate constants were calculated for the temperature range 200-2000 K using the three methods and compared with available experimental data. We found that (a) the total reaction probabilities are very small, (b) the symmetric and asymmetric N-H stretch excitations enhance the reactivity, (c) the quantum-mechanical calculated thermal rate constants are about one order of magnitude smaller than the transition state theory results, which reproduce the experimental evidence, and (d) quasi-classical trajectory calculations, which were performed with the main goal of analyzing the influence of the zero-point energy problem on the final dynamics results, reproduce the quantum scattering calculations on the same surface. [ABSTRACT FROM AUTHOR]

A new continuous configuration time-dependent self-consistent field method has been developed to study polyatomic dynamical problems by using the discrete variable representation for the reaction system, and applied to a reaction system coupled to a bath. The method is very efficient because the equations involved are as simple as those in the traditional single configuration approach, and can account for the correlations between the reaction system and bath modes rather well. [ABSTRACT FROM AUTHOR]

The exact description of the prototypical X + CH4 reactions in the framework of quantum mechanics is a long-standing challenge in chemical reaction dynamics. A number of reduced-dimensional models have been developed and advanced our understanding of polyatomic chemical reactivity. Here, we propose a new ten-dimensional (10D) quantum dynamics model for the kind of X + YCAB2 reaction. The new model is an extension of our previous eight-dimensional (8D) model for the X + YCZ3 reaction. In this 10D model, AB2 holds C2v symmetry and the group CAB2 is assumed to keep Cs symmetry, which gives a better description of the rocking mode of methane than the previous 8D model. The reaction dynamics of H + CH4 → H2 + CH3 is first investigated using this model. The calculations showed that the rocking mode has a similar reaction efficiency to the umbrella mode. For the degenerate asymmetric stretching mode, the averaged efficiency is comparable to that of the symmetric stretching mode. [ABSTRACT FROM AUTHOR]

We employ few-cycle pulses to strong-field-ionize di-iodomethane (CH2I2) and femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the subsequent ultrafast dissociative ionization and vibrational wave packet dynamics. Probing in the spectral region of the I 4d core-level transitions, the time-resolved XUV differential absorption spectra reveal the population of several electronic states of CH2I2+ by strong-field ionization. Global analysis reveals three distinct time scales for the observed dynamics: 20 ± 2 fs, 49 ± 6 fs, and 157 ± 9 fs, ascribed to relaxation of the CH2I2+ parent ion from the Franck-Condon region, dissociation of high-lying excited states of CH2I2+ to I+ (3P2), CH2I, and I2+ (2Π3/2,g), and dissociation of CH2I2+ to I (2P3/2) and CH2I+, respectively. Oscillatory features in the time-resolved XUV differential absorption spectra point to the generation of vibrational wave packets in both the residual CH2I2 and the CH2I2+ parent ion. Analysis of the oscillation frequencies and phases reveals, in the case of neutral CH2I2, C–I symmetric stretching induced by bond softening and I–C–I bending driven by a combination of bond softening and R-selective depletion. In the case of CH2I2+, both the fundamental and first overtone frequencies of the I–C–I bending mode are observed, indicating large-amplitude I–C–I bending motion, in good agreement with results obtained from ab initio simulations of the XUV transition energy along the I–C–I bend coordinate. These results show that femtosecond XUV absorption spectroscopy is well-suited for studying ultrafast photodissociation and vibrational wave packet dynamics. [ABSTRACT FROM AUTHOR]

The proton transfer reaction H3+ + CO → H2 + HCO+/HOC+ is of great importance in interstellar and circumstellar environments. The dynamics and kinetics of the reaction were studied using the quasiclassical trajectory approach on a newly developed ab initio potential energy surface (PES) for the ground electronic state. The PES was constructed by fitting 46 462 ab initio points at the level of CCSD(T)-F12a/aug-cc-pVTZ using the fundamental invariant-neural network method. The calculated product branching ratio, scattering angular distribution, and thermal rate coefficient agreed reasonably well with the experimental measurements, which were rationalized by the underlying reaction mechanisms. [ABSTRACT FROM AUTHOR]

An eight-dimensional quantum mechanical Hamiltonian has been proposed based on Palma and Clary's model in which the non-reacting CZ3 group keeps a C3v symmetry in the X + YCZ3 ↔ XY + CZ3 reaction J. Palma and D. C. Clary [J. Chem. Phys. 112, 1859 (2000)]. By transforming the original Cartesian coordinate system (x, s) into a scaled polar coordinate system (q, γ), the vibrational Hamiltonian of CZ3 group is expressed in a simple form with a clear physical picture. This Hamiltonian is used to investigate the H + CH4 → H2 + CH3 reaction on the Jordan-Gilbert potential energy surface. The total reaction probabilities are calculated for the initial ground state, and umbrella, bending, symmetric, and asymmetric stretching excited states of CH4 with total angular momentum J = 0. The integral cross sections for the reaction are also studied for these initial vibrational states with a centrifugal-sudden approximation. The total integral cross sections for the asymmetric stretching vibrational excited state are in good agreement with the experimental observations. The results also showed the difference of dynamical behavior between reactions from symmetric and asymmetric stretching excited states. The thermal rate constants are calculated for the temperature range T = 250-2000 K and compared with the experimental and other theoretical results. [ABSTRACT FROM AUTHOR]

The dynamics of electronically excited states in 2-picoline is studied using femtosecond time-resolved photoelectron imaging spectroscopy. The internal conversion from the S2 state to the vibrationally excited S1 state is observed in real time. The secondarily populated high vibronic S1 state deactivates further to the S0 state. Photoelectron energy and angular distributions reveal the feature of ionization from the singlet 3p Rydberg states. In addition, variation of time-dependent anisotropy parameters indicates the rotational coherence of the molecule. [ABSTRACT FROM AUTHOR]

Theoretical studies of the potential energy surface and bound states were performed for the N2O dimer. A four-dimensional intermolecular potential energy surface (PES) was constructed at the CCSD(T) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to a nonpolar isomer with slipped-antiparallel planar structure and two equivalent polar isomers with slipped-parallel planar structures. The nonpolar isomer is energetically more stable than the polar ones by 162 cm-1. To assign the fundamental vibrational frequencies for both isomers, more than 150 vibrational bound states were calculated based on this PES. The orientation of the nodal surface of the wave functions plays an important role in the assignment of disrotation and conrotation vibrational modes. The calculated vibrational frequencies are in good agreement with the available experimental data. We have also found a quantum tunneling effect between the two equivalent polar structures in the higher vibrational excited states. Rotational transition frequencies of the polar structure were also calculated. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters. [ABSTRACT FROM AUTHOR]

A large angle and high linearity two-dimensional laser scanner with an in-house ingenious deflection angle detecting system is developed based on voice coil actuators direct driving mechanism. The specially designed voice coil actuators make the steering mirror moving at a sufficiently large angle. Frequency sweep method based on virtual instruments is employed to achieve the natural frequency of the laser scanner. The response shows that the performance of the laser scanner is limited by the mechanical resonances. The closed-loop controller based on mathematical model is used to reduce the oscillation of the laser scanner at resonance frequency. To design a qualified controller, the model of the laser scanner is set up. The transfer function of the model is identified with MATLAB according to the tested data. After introducing of the controller, the nonlinearity decreases from 13.75% to 2.67% at 50 Hz. The laser scanner also has other advantages such as large deflection mirror, small mechanical structure, and high scanning speed. [ABSTRACT FROM AUTHOR]

Theoretical studies of the potential energy surface and bound states were performed for the CO2-N2O van der Waals complex. A four-dimensional intermolecular potential energy surface (PES) was constructed from 11 466 ab initio data points which were calculated at the coupled-cluster single double (triple) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to two equivalent isomers with a slipped parallel structure in which the O atom in N2O is near the C atom in CO2 and a T-shaped isomer in which the terminal N atom in N2O is closest to the C atom in CO2. The two slipped parallel isomers are energetically more stable than the T-shaped isomer by 178 cm-1. Four fundamental vibrational excited states for the slipped parallel isomers and two fundamental vibrational excited states (torsion and disrotation) for the T-shaped isomer were assigned via bound states calculations based on this PES. The theoretical vibrational frequencies are in good agreement with the available experimental values for the slipped parallel isomers. Rotational excitations (J = 0-6) for the ground vibrational state of the slipped parallel structure were calculated and the accuracy of the PES in the vicinity of minima is validated by the good agreement between the theoretical and experimental transition frequencies and spectroscopic parameters. [ABSTRACT FROM AUTHOR]