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Several N-heterocyclic carbenes (NHCs) are experimentally observed upon the addition of muonium (Mu), and the hyperfine coupling constants (HFCCs) of muon are measured. Theoretical investigation of Mu has been challenging due to significant quantum effects. Herein, we performed an ab initio path integral molecular dynamics (PIMD) simulation, which accurately considers multi-dimensional quantum effects, to theoretically investigate muoniated 1,3-dihydro-2H-imidazole-2-ylidene (Mu-IY). Our findings indicate that quantum effects have two contradictory contributions: the quantum effect of bond vibrations increases the HFCC values, whereas that of out-of-plane angular vibrations decreases the HFCC values. Moreover, we show that the HFCC values of other NHCs can be predicted without the PIMD simulations by applying the structural changes caused by the quantum effect derived from the PIMD simulations of Mu-IY. [ABSTRACT FROM AUTHOR]

We propose an efficient algorithm for a minimum free energy path calculation based on the path integral hybrid Monte Carlo (PIHMC) method by combining the climbing image-nudged elastic band (CI-NEB) and the thermodynamic integration (TI) methods. Here, the CI-NEB and the TI methods are used to find a transition state along the reaction path and evaluate the free energy path, respectively. Our algorithm is applied to the Walden inversion reaction of the hydronium ions (H3O+). The numerical results show that the computational effort by our algorithm is significantly reduced compared to that of the previously proposed algorithm combining PIHMC without losing accuracy. We also demonstrate the importance of temperature and isotope effects on the addition reaction of hydrogen isocyanide to formaldehyde. In this reaction, the nuclear quantum effect causes the structural change at the TS and decreases the energy barrier. [ABSTRACT FROM AUTHOR]

This study explores various models of semiconductor dielectric functions, with a specific emphasis on the large wavenumber spectrum and the derivation of the screened exchange interaction. Particularly, we discuss the short-range effect of the screened exchange potential. Our investigation reveals that the short-range effect originating from the high wavenumber spectrum is contingent upon the dielectric constant of the targeted system. To incorporate dielectric-dependent behaviors concerning the short-range aspect into the dielectric density functional theory (DFT) framework, we utilize the local Slater term and the Yukawa-type term, adjusting the ratio between these terms based on the dielectric constant. Additionally, we demonstrate the efficacy of the time-dependent dielectric DFT method in accurately characterizing the electronic structure of excited states in dyes and functional molecules. Several theoretical approaches have incorporated parameters dependent on the system to elucidate short-range exchange interactions. Our theoretical analysis and discussions will be useful for those studies. [ABSTRACT FROM AUTHOR]

The theoretical modeling of high-pressure ice remains challenging owing to the complexity in accurately reflecting its properties attributable to nuclear quantum effects. To explore the nuclear quantum effects of the phase transition between Ice VII and Ice X, we introduce an approach based on ab initio path-integral molecular dynamics. The results indicate that quantum effects facilitate the phase transition, with the observed isotope effects consistent with the experimental outcomes. We demonstrate that quantum effects manifest differently across ice phases: In Ice VII, quantum effects reduce the pressure through the centralization of protons. In contrast, in Ice X, quantum effects increase the pressure owing to the increased kinetic energy of zero-point vibration. [ABSTRACT FROM AUTHOR]

Deuterium isotope effects on interaction energies and geometrical parameters in several H3O+(D3O+)...ene and H3O+(D3O+)...yne complexes, which involve O‐H(D)...π interactions, have been analyzed using the MP2 level of the multi‐component molecular orbital method (MC_MP2), which can incorporate the nuclear quantum effects of light nuclei, such as protons and deuterons. The MC_MP2 calculations revealed that D3O+ replacement reduced the interaction energies of the complexes and induced changes in geometrical parameters. In addition, natural energy decomposition analysis (NEDA) revealed a strong correlation between the H/D isotope effects on the H/D...π distances and on each energy component. [ABSTRACT FROM AUTHOR]

A positive Mu is a useful tool for investigating the spin density of radical species. The theoretical estimation of its behavior in a molecule requires the inclusion of a quantum effect due to the small mass of muonium. Herein, we performed ab initio a path integral molecular dynamics (PIMD) simulation, which accurately included a multi-dimensional quantum effect, for muoniated 9H-xanthene-9-thione (μXT). Our results showed that the quantum effect significantly increased the hyperfine coupling constant (HFCC) value of μXT, which qualitatively improved the calculated HFCC value, compared to the experimental one. In the PIMD simulation, the bond length between muonium and sulfur in μXT is longer than that between hydrogen and sulfur in a hydrogenated 9H-xanthene-9-thione (HXT), leading to a spin density transfer from XT (9H-xanthene-9-thione) to muonium due to neutral dissociations. Additionally, we found that the S–Mu bond in μXT prefers a structure perpendicular to the molecular plane, where the interaction between Mu and the singly occupied molecular orbital of μXT is the strongest. These structural changes resulted in a larger HFCC value in the PIMD simulation of μXT. [ABSTRACT FROM AUTHOR]

We studied the stability of a system consisting of a positron (e+) and two lithium anions, [Li−; e+; Li−], using first-principles quantum Monte Carlo calculations combined with the multi-component molecular orbital method. While diatomic lithium molecular dianions Li 2 2 − are unstable, we found that its positronic complex can form a bound state with respect to the lowest energy decay into the dissociation channel Li 2 − and a positronium (Ps). The [Li−; e+; Li−] system has the minimum energy at the internuclear distance of ∼3 Å, which is close to the equilibrium internuclear distance of Li 2 − . At the minimum energy structure both an excess electron and a positron are delocalized as orbiting around the Li 2 − molecular anion core. A dominant feature of such a positron bonding structure is described as the Ps fraction bound to Li 2 − , unlike the covalent positron bonding scheme for the electronically isovalent [H−; e+; H−] complex. [ABSTRACT FROM AUTHOR]

We focus on the unique aspects of biuret and biguanide, which form six-membered ring structures via intramolecular hydrogen bonds. The proton donor and acceptor atoms differ between biuret and biguanide, leading to varying energy barrier heights for proton transfer. We performed path integral molecular dynamics (PIMD) simulations for biuret and biguanide to investigate the correlation between proton transfer and the degree of the delocalization of π-electrons in the six-membered ring framework structure. The results indicate that the π-electrons in the framework structure are delocalized regardless of the ease of intramolecular proton transfer. [ABSTRACT FROM AUTHOR]

Aromaticity is an important concept in organic chemistry, and thus, many theoretical and experimental studies have been conducted so far. However, the majority of theoretical studies have concentrated on the aromaticity of the stationary point structures. Herein, the influence of nuclear quantum fluctuation (nuclear quantum effects: NQEs) and thermal fluctuation on the aromaticity of benzene have been analyzed by path integral molecular dynamics (PIMD) simulation. The PIMD simulations revealed that the NQEs affected not only the C–H bonds but also the C–C bonds. The HOMA and NICS calculations demonstrated that the aromaticity decreased due to the NQEs of carbon atoms, attributed to an increase in the contribution from specific vibrational modes strongly correlated with benzene's aromaticity. [ABSTRACT FROM AUTHOR]