Journal of Chemical Physics最新文献

Phosphine (PH3) is the simplest phosphorus compound detected in the extrasolar and planetary environments, where it can be subjected to ionizing radiation. In this study, we first report the vibrational spectra of the ionized molecules resulting from phosphine monomer and dimer in solid noble gas matrices upon X-ray irradiation. The assignment was based on the comparative studies using electron paramagnetic resonance and Fourier transform infrared spectroscopy, complemented by the quantum-chemical calculations at the valence-correlated spin unrestricted coupled cluster single-double and perturbative triple [UCCSD(T)] level of theory. We were able to observe three fundamentals of PH3+• and four fundamentals of P2H6+•. The spectroscopic data were also obtained for PD3+•, P2D6+•, and [PH3-PD3]+• radical cations. PH3+• demonstrates a large blue shift of the most intense infrared (IR) absorption related to the P-H stretching vibration (ν1) with respect to the corresponding vibration in the parent neutral. For the P2H6+• radical cation, the most intense IR absorption corresponds to a low-frequency deformational vibration. It was found that the P2H6+• radical cation reveals a photochromic behavior: it decays under the action of light with λ ≤ 400 nm and can be partially recovered after subsequent photolysis at λ = 445-525 nm. The observed transformations were attributed to the interconversion between P2H6+• and the PH4+…PH2• complex. The dynamics of these transformations was discussed using the data obtained for mixed P2H3D3+• species. The obtained results may be useful for future searches for the manifestations of such species in extraterrestrial environments and studies of the radiation-induced transformations of phosphine in icy media.

In this article, the Kosambi-Cartan-Chern (KCC) theory, which uses geometric invariants to characterize the time evolution of systems of ordinary differential equations, is employed for systems of ordinary differential equations describing chemical oscillations. This study demonstrates the utility of the KCC theory in providing a geometric framework for stability analysis, highlighting its ability to complement traditional Lyapunov stability assessments.

We developed an 8D, site-specific, reduced-dimensionality potential energy surface (PES) for H2 dissociation at the ortho site of hexagonal BN, treating two surface atom degrees of freedom explicitly. To our knowledge, this is the first PES for gas-phase dissociation on a covalent surface with explicit treatment of surface atom motion and symmetry in a reduced-dimensionality approach. The PES was fitted to 16 164 PBE+D3 DFT data points with a mean absolute error of 17.00 meV using a symmetry-aware neural network incorporating atom permutational invariance and lattice space group symmetry. Convergence was validated via 2D site vibrational Schrödinger equation integration and classical MD trajectory prediction uncertainty analysis. Comparison of site-specific vibrations with the surface phonon spectrum revealed the limitations of the phonon approximation for covalent PES generation using EAM-thermal averaging methods. Classical trajectory analysis reveals that H-H bond elongation beyond 1.5 Å occurs at incident energies as low as 2.25 eV, which is below the 2.664 eV chemisorption barrier, suggesting BN surface mobility facilitates pre-dissociation. Our 8D model reproduces the barrier with 17% error (0.387 eV) compared to the full 108-dimensional model, while providing at least four orders of magnitude computational speedup compared to other surface models, enabling wavepacket dynamics calculations. This methodology provides a framework for future quantum dynamics studies on covalent 2D materials, using site vibrations to construct efficient discrete-variable representation basis sets.

Equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) provides a robust framework for describing a wide range of electronically excited and open-shell states. Among its various formulations, the double electron-attachment (EOM-DEA-CCSD) and double ionization potential (EOM-DIP-CCSD) methods are particularly effective for treating diradicals and other types of open-shell species. To enable accurate geometry optimizations and property calculations, we present the derivation and implementation of analytic nuclear gradients for EOM-DEA-CCSD and EOM-DIP-CCSD. These new capabilities were illustrated by calculations of singlet-triplet gaps in benzyne diradicals as well as characterization of molecules relevant to quantum information science. We further extended the framework to include spin-orbit coupling calculations for EOM-DEA-CCSD states, allowing calculations of intensity borrowing and intersystem crossings.

We report predictions and measurements of O2 absorption spectra that exhibit line intensity depletion with increasing gas density. This effect, which is attributed to the finite duration of collisions, alters the line shape by redistributing a portion of the intensity from a relatively narrow spectrum that can be described by an impact-approximation-based profile to a broad pedestal with a width that is inversely related to the collision duration. Using classical molecular dynamics simulations (CMDS), we predicted details regarding this mechanism for O2 with four collision partners: O2, N2, Ar, and He at a temperature of 296 K. These simulations were validated by comparisons with experimental intensity depletion coefficients obtained from absorption spectra of the 1.27 μm band of O2 in air; Ar and He acquired over a wide pressure range up to 120 kPa. All experimental spectra were recorded using high-precision cavity ring-down spectroscopy (CRDS) apparatuses at NIST (United States of America) and LIPhy (France). For air-broadened O2, more specifically, a mean depletion value of ∼0.3% amagat-1 was observed, with almost no resolvable rotational dependence. The temperature dependence of the intensity depletion in this system was also investigated by CMDS at 250 and 296 K and by CRDS spectra of air at 250, 275, and 296 K. The theoretical results suggest a nearly 1/T2 temperature dependence of the intensity-weighted depletion coefficient, which over the limited temperature range considered, was only slightly greater than the measurement precision. Finally, simulations of atmospheric solar absorption spectra were implemented to quantify the impact of neglecting this depletion effect on the retrieved surface pressure, resulting in a negatively biased measurement of ∼0.14%, with a spread of ∼0.02% caused by seasonal variations in gas temperature.

The universal phase behavior of block copolymer melts demonstrated previously with particle-based simulations is reproduced using complex-Langevin field-theoretic simulations (CL-FTSs) combined with the Morse calibration. For comparison purposes, the calculations are repeated using conventional Langevin field-theoretic simulations (L-FTSs), where the partial saddle-point approximation (PSPA) is applied to the pressure field. Both FTS methods produce consistent results down to invariant polymerization indices of N̄≈105, implying that the inaccuracies in the PSPA are well compensated for by the Morse calibration. At lower N̄, however, the complex fields of the CL-FTSs become prone to the formation of hot spots, causing the simulations to fail. Previous studies have shown that finite-range interactions can help stabilize CL-FTSs. Aided by the L-FTSs, we locate conditions at N̄=104, under which the universality is expected to hold and the CL-FTSs are stable. While the L-FTSs continue to obey universality, the CL-FTSs deviate significantly. A number of potential explanations are considered, but only one appears credible. Given the documented problems with CL simulations of nonpolymeric models, it is likely that the inconsistency with universality results from a "silent failure" in the CL-FTSs, preceding the formation of hot spots.

The diffusion Monte Carlo (DMC) method is generally considered less sensitive to basis set incompleteness than conventional electronic structure approaches. Its performance for systems containing second-row elements in high oxidation states-where tight d-functions play an essential role-remains insufficiently explored. In this work, we investigate the influence of basis sets and high-exponent d-functions on atomization energies of molecules containing first- and second-row elements using both DMC and CCSD(T) with correlation-consistent effective core potentials. Tight d-functions are found to significantly affect the nodal structure of the trial wave functions, particularly for molecules containing second-row elements in high oxidation states, thereby impacting the DMC energies. For molecules containing second-row elements, at least the a(T+d)Z or aQZ basis set is required to obtain accurate results, whereas molecules containing second-row elements in high oxidation states demand even larger sets such as a(Q+d)Z or a5Z. Moreover, basis set effects on DMC energies are approximately half those observed at the HF level, with a strong correlation between the two, suggesting that HF calculations can provide useful guidance for basis set selection in DMC. These findings highlight the critical role of tight d-functions in ensuring reliable DMC predictions for chemically challenging systems.

We present an algorithm for finding chemical reaction pathways using a Monte Carlo transition state search (MCTSS) scheme. Our strategy is a bidirectional two-state approach that simultaneously drives two Monte Carlo trajectories from reactants to products, and vice versa, until the trajectories meet. The trajectories are driven in a Metropolis-like procedure with transition probabilities based on the real-space diffusion Monte Carlo algorithm. A computationally inexpensive structure preselection procedure is used to guide the two trajectories toward each other. We performed a proof-of-principle demonstration of the MCTSS algorithm for the model two-dimensional double-well potential and for the halogen anion SN2-substitution in halogenated methane. The MCTSS approach presented here is expected to be particularly useful when employing electronic structure methods that do not provide analytic gradients.

Developing analytic equations of state for fluid mixtures based on perturbation theories requires simplifying approximations, in which mixture properties are determined from effective pure component properties. While these one-fluid approximations reduce model complexity, they also introduce inaccuracies. In this work, a simple algebraic correction factor for perturbation theories applying a one-fluid approximation is proposed. The correction factor is defined as the ratio of the rigorous first-order perturbation term in Helmholtz energy to the respective first-order perturbation term in the one-fluid approximation. An approximate closed-form expression for the correction factor is derived in terms of the fundamental measures of the particles using symbolic regression. The new approach is thoroughly evaluated by applying it to a range of equations of state, including the uv-theory and the PCP-SAFT equation of state, to predict the thermodynamic properties of square-well, Lennard-Jones, and real-substance mixtures. New molecular simulation data for strongly size-asymmetric binary, ternary, and quinary Lennard-Jones mixtures are generated to test the proposed approach. Compared to the predictions obtained from a one-fluid approach, the correction factor significantly improves the accuracy of predicted phase equilibria and thermodynamic properties for model fluids. Its impact on real-fluid predictions with the PCP-SAFT equation of state is rather small, because PCP-SAFT segment size parameters of most substances are rather similar, whereas the correction factor primarily accounts for segment size asymmetry.

We show how to adapt and improve the stochastic simulation algorithm (SSA), also known as the Lanore-Gillespie algorithm, to exactly and efficiently simulate a large, fully connected network of chemical reactions. By combining a low-rank decomposition of an upper bound of the propensity matrix with rejection sampling, we are able to significantly reduce the time and memory costs of manipulating the reactions' priority queues. The resulting algorithms exhibit logarithmic time and linear space complexity in the number of involved chemical species, outperforming the original SSA and subsequent stochastic methods on a benchmarking model. As a physical application, we simulate the time evolution of solute precipitation in a FeCu1.34% alloy under thermal aging. The substantial speed-up and significantly reduced memory consumption enable us to reach physical times and system sizes that were unattainable with previously employed deterministic and stochastic methods. The temporal evolution of the simulated sizes and number densities of Cu precipitates also matches very well with small-angle neutron scattering experiments.