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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 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]

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 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]