Positrons, the antiparticles of electrons, serve as unique probes for fundamental interactions and are crucial in diverse applications. We present a new mechanism in chemical bonding: the formation of a positron-driven bond that fundamentally alters electronic bonding interactions. Investigating the Be₂ dimer with Quantum Monte Carlo (QMC) simulations, we construct the potential energy curve of e⁺:Be₂. Our analysis reveals a significant energetic stabilization of the Be–Be bond upon positron attachment, a result that challenges conventional understanding. We show this stabilization arises from a novel, two-stage mechanism: at longer distances, a positron bond forms via internuclear positron accumulation, similar to that in positron–anion systems. However, as the atoms approach equilibrium, the positron density undergoes a unique redistribution, moving out of the internuclear region to accumulate in the outer molecular vicinity. This distinct positron localization, combined with an otherwise repulsive electronic component, leads to overall system stabilization as the electron density dynamically follows the positron. This work expands our understanding of chemical bonding by strongly suggesting how an antiparticle can profoundly influence molecular stability.

We introduce a non-orthogonal configuration interaction approach to investigate nuclear quantum effects on energies and densities of confined fermionic nuclei. The Hamiltonian employed draws parallels between confined systems and many-electron atoms, where effective non-Coulombic potentials represent the interactions of the trapped particles. One advantage of this method is its generality, as it offers the potential to study the nuclear quantum effects of various confined species affected by effective isotropic or anisotropic potentials. As a first application, we analyze the quantum states of two ³He atoms encapsulated in C₆₀. At the Hartree–Fock level, we observe the breaking of spin and spatial symmetries. To ensure wavefunctions with the correct symmetries, we mix the broken-symmetry Hartree–Fock states within the non-orthogonal configuration interaction expansion. Our proposed approach predicts singly and triply degenerate ground states for the singlet (para-³He₂@C₆₀) and triplet (ortho-³He₂@C₆₀) nuclear spin configurations, respectively. The ortho-³He₂@C₆₀ ground state is 5.69 cm⁻¹ higher in energy than the para-³He₂@C₆₀ ground state. The nuclear densities obtained for these states exhibit the icosahedral symmetry of the C₆₀ embedding potential. Importantly, our calculated energies for the lowest 85 states are in close agreement with perturbation theory results based on a harmonic oscillator plus rigid rotor model of ³He₂@C₆₀. 

We provide experimental evidence that is inconsistent with often proposed Langmuir−Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au–Fe₂O₃. Passing CO and H₂O, but no O₂, over Au-γ-Fe₂O₃ at 25 °C, we observe significant CO₂ production, inconsistent with LH mechanistic hypotheses. Experiments with H₂¹⁸O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe₂O₃. Guided by density functional theory, we instead postulate a water-promoted Mars–van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO₂ production in the absence of O₂ and with CO oxidation in the presence of H₂¹⁸O and ¹⁶O₂. In contrast, for Au-TiO₂, our data is consistent with previous LH mechanistic hypotheses. 

Positronium (Ps) exhibits the ability to form energetically stable complexes with atoms and molecules before annihilation occurs. In particular, F, a halogen, shows the highest reported positronium binding energy (2.95 eV) in the periodic table. Superhalogens are defined as molecules with electron affinities exceeding that of Cl (3.61 eV), the atom with the highest electron affinity. Building upon the concept of superhalogens, we can define Ps-superhalogens as molecules with Ps binding energies surpassing that of F. This study explores structural and energetic aspects of positronium and positron binding to neutral and anionic superhalogen molecules of the MX_k+1 family (M = Li, Na, Be, Mg, B, Al, Si, P; X = F, Cl, Br), respectively and where k represents the highest formal valence of M. We perform multicomponent MP2 calculations for positron systems, which reveal how positron affinities vary with the type and number of halogen atoms present. The analysis of the results emphasizes the predominant role of electrostatic interactions in determining the positron affinity, with negligible effects of electronic and geometric relaxation upon positron attachment. We predict the energetic stability of 22 of the 24 PsMX_k+1 complexes with respect to the chemically relevant dissociation channels: e⁺ emission, Ps emission and M–X bond breaking. Our findings reveal six MF_k+1 systems that qualify as Ps-superhalogens, showing a positronium binding energy exceeding 2.95 eV. Of these, AlF₄ stands out by setting a new record for the highest positronium binding energy among neutral molecules, reaching 4.36 eV.

This study explores the energetic stability and physical properties of (Formula presented.) complexes formed by two halide anions (X−, Y−=F−, X−, Br−), and two positrons (Ps: positron-electron pair). We combine electronic coupled cluster (CCSD(T)) calculations with positronic multicomponent renormalized partial third-order propagator (MC-REN-PP3) calculations to effectively recover correlation energies. Analysis of potential energy curves confirms the energetic stability of these positronic molecules, with optimized structures identified as global minima. Further investigation of electron and positron densities reveals stabilization owing to the formation of two-positron bonds. The global stability of the (Formula presented.) complexes contrasts with the metastable two-positron-bonded (PsH)2, which energetically favors the emission of Ps2. Comparative analysis of one- and two-positron dihalides indicates that the addition of a positron to PsXY− generally results in shorter bond distances, higher force constants, and lower dissociation energies, with exceptions due to differences in positron affinities of PsXY− and Y−. We explore the analogy between two-positron-bonded dihalide systems (Formula presented.) and two-electron-bonded dialkali molecules AB, (A, B=Na, K, Rb). The bonding properties in two-positron dihalides and their electronic dialkali analogs are comparable, displaying identical periodic trends. However, compared to their isoelectronic AB counterparts, the positron bonds in (Formula presented.) have shorter bond lengths, higher force constants, and higher bond energies.

We summarize in this article the recent progress made in our laboratories in the development of numerical approaches dedicated to investigating ultrafast physicochemical responses of biological matter subjected to ionizing radiations. Our modules are integrated into the deMon2k software which is a readily available program with highly optimized algorithms for conducting Auxiliary Density Functional Theory (ADFT) calculations. We have developed a computational framework based on Real-Time Time-dependent ADFT to simulate the electronic responses of molecular systems to strong perturbations, while molecular dynamics simulations in the ground and excited states (Ehrenfest dynamics) are available to simulate irradiation-induced ultrafast bond breaking/formation. Constrained ADFT and Multi-component ADFT have also been incorporated to simulate charge transfer processes and nuclear quantum effects, respectively. Finally, a coupling to polarizable force fields further permits to realistically account for the electrostatic effects that the systems' environment has on the perturbed electron density. The code runs on CPU or hybrid CPU/GPU architectures affording simulations of systems comprised up to 1000 atoms at the DFT level with controlled numerical accuracy. We illustrate the applications of these methodologies by taking results from our recent articles that aimed principally at understanding experimental data from pulse radiolysis experiments.

This work presents an extension of the projector operator embedding scheme of Manby et al. [J. Chem. Theory Comput. 8, 2564 (2012)] in a multicomponent (MC) framework. Here, a molecular system containing electrons and other types of quantum species is divided into a wavefunction (WF) subsystem of interest and a density functional theory (DFT) environment. The WF-in-DFT partition decreases computational costs by partially truncating the WF subsystem basis set at the cost of introducing a controllable embedding error. To explore the applicability of the MC extension, third-order propagator-in-DFT calculations were performed for positron–anion complexes for alkoxides and carboxylates with carbon chains of different sizes. For these systems, it was found that selecting a WF subsystem with the positron and only the oxygen atoms caused an error of 0.1 eV or lower in positron-binding energies, while reducing between 33% and 55% the basis set size. The reduction of computational costs achieved with the embedding scheme allowed us to improve molecular positron-binding energy predictions by performing complete basis set limit extrapolations. Combining the WF-in-DFT embedding and the complete basis set extrapolation, positronium aliphatic alkoxides were predicted to be energetically stable by 0.3 eV with respect to Ps emission. Similarly, positronium carboxylates, both aromatic and aliphatic, were predicted to be stable by 1.3 eV.

Computational studies have shown that one or more positrons can stabilize two repelling atomic anions through the formation of two-center positronic bonds. In the present work, we study the energetic stability of a system containing two positrons and three hydride anions, namely 2e⁺[H₃³⁻]. To this aim, we performed a preliminary scan of the potential energy surface of the system with both electrons and positrons in a spin singlet state, with a multi-component MP2 method, that was further refined with variational and diffusion Monte Carlo calculations, and confirmed an equilibrium geometry with D3h symmetry. The local stability of 2e⁺[H₃³⁻] is demonstrated by analyzing the vertical detachment and adiabatic energy dissociation channels. Bonding properties of the positronic compound, such as the equilibrium interatomic distances, force constants, dissociation energies, and bonding densities are compared with those of the purely electronic H₃⁺ and Li₃⁺ systems. Through this analysis, we find compelling similarities between the 2e⁺[H₃³⁻] compound and the trilithium cation. Our results strongly point out the formation of a non-electronic three-center two-positron bond, analogous to the well-known three-center two-electron counterparts, which is fundamentally distinct from the two-center two-positron bond [D. Bressanini, J. Chem. Phys., 2021, 155, 054306], thus extending the concept of positron bonded molecules.