Márcio Teixeira Do Nascimento Varella

We present a computational investigation into the anion states of 2,4-dichloro-5-nitropyrimidine (NDP), focusing on dissociative electron attachment pathways motivated by recent mass spectrometry experiments. The molecule exhibits a unique combination of low-energy electron-induced mechanisms potentially relevant to synergistic effects in concurrent chemo-radiation cancer therapy. Our calculations reveal two bound states (π1* and π2*), three shape resonances (π3*, σCCl1*, and σCCl2*), and a π4* resonance of mixed shape and core-excited character. We attribute the intense fragmentation, [M–NO]− and [M–NO2]−, and the parent anion state at 0 eV mainly to the π3* state, characterized as either a weakly bound state or a resonance slightly above threshold. While such processes could in principle occur via vibrational Feshbach resonances, the π2* state is too deeply bound (at least by 0.71 eV) to enable this mechanism. The complex fragmentation induced by nearly zero energy electrons would instead arise from nonadiabatic interactions between the π3* state and the lower-lying bound states π1* and π2*. This coupling between bound anion states and a low-energy resonance creates pathways for internal conversion and dissociation and could be viewed as a target property for radiosensitization.

Mixed quantum–classical dynamics (MQCD) methods are effective models for excited-state processes in quasi-classical molecular systems, in which nuclear motion is described by classical trajectories while electronic populations undergo quantum nonadiabatic transitions. This article presents Newton-X 26, a new generation of the Newton-X platform that consolidates two decades of development into a modular ecosystem for generating spectra and initial conditions, propagating dynamics, and analyzing, postprocessing, and archiving data. Newton-X 26 supports multiple MQCD strategies, including surface hopping, decoherence-corrected Ehrenfest dynamics, and ab initio multiple spawning, and connects to a range of electronic-structure engines through dedicated interfaces. The platform emphasizes efficient execution for large trajectory ensembles, enabling systematic convergence analyses and uncertainty estimation. Complementary tools support automated data curation, machine-learning-assisted workflows, and reproducible FAIR-oriented reporting and sharing. Taken together, Newton-X 26 provides an open-source environment for routine MQCD applications and continued method development across multiple electronic-structure levels.

Mixed quantum–classical dynamics (MQCD) methods are effective models for excited-state processes in quasi-classical molecular systems, in which nuclear motion is described by classical trajectories while electronic populations undergo quantum nonadiabatic transitions. This article presents Newton-X 26, a new generation of the Newton-X platform that consolidates two decades of development into a modular ecosystem for generating spectra and initial conditions, propagating dynamics, and analyzing, postprocessing, and archiving data. Newton-X 26 supports multiple MQCD strategies, including surface hopping, decoherence-corrected Ehrenfest dynamics, and ab initio multiple spawning, and connects to a range of electronic-structure engines through dedicated interfaces. The platform emphasizes efficient execution for large trajectory ensembles, enabling systematic convergence analyses and uncertainty estimation. Complementary tools support automated data curation, machine-learning-assisted workflows, and reproducible FAIR-oriented reporting and sharing. Taken together, Newton-X 26 provides an open-source environment for routine MQCD applications and continued method development across multiple electronic- structure levels.

Surface hopping simulations critically depend on the accuracy and robustness of the underlying electronic structure methods. Fully correlated approaches─such as CASPT2, MRCI, L-PDFT, and MRSF-TDDFT─that account for both dynamic and static electron correlation (without implying an exact treatment of electron correlation) offer significant promise. Still, their practical application in dynamics remains limited by the computational cost and technical challenges. In this Perspective, we examine the current status of such methods by analyzing representative surface-hopping simulations of fulvene and pyrrole, two prototypical systems for photophysical and photochemical processes. These examples demonstrate that while fully correlated methods improve the description of bond rearrangements and hot ground-state dynamics, partially correlated approaches─such as ADC(2) and TDDFT─remain sufficient for photophysical excited-state relaxation. Across methods, persistent limitations, such as active-space instabilities and potential-energy discontinuities, imply the need for improved approaches. We argue that expanding the use of generalized active spaces in the short term and advancing large active space algorithms in the long term will be crucial for making high-accuracy nonadiabatic simulations broadly reliable and accessible.

By overcoming ‘Kappaphobia’, we show statistical approximations underestimate κ ² in high-efficiency organic photovoltaic materials.

Positronium (Ps) atoms are highly sensitive probes of condensed-phase structure and dynamics, yet their theoretical description in complex molecular environments remains challenging. We present an extension of our QM/MM protocol to investigate Ps in methanol and ethanol, building on our earlier study in water. Classical Monte Carlo simulations employing newly parametrized Ps-solvent force fields reveal well-defined cavity structures, whose sizes are consistent with hydrated Ps and systematically smaller than those predicted by empirical bubble models. Multicomponent quantum calculations employing the Any Particle Molecular Orbital (APMO) method have identified physically meaningful cavity states, characterized by substantial electron-positron overlap, and have ruled out spurious surface states as artifacts of the QM region truncation. While vertical detachment energies are insensitive primarily to solvent structure, computed pick-off annihilation lifetimes showed a marked dependence on cavity size. Employing orbital-dependent enhancement factors, our results achieve good agreement with experimental PALS data for both solvents. This study demonstrates the transferability and predictive power of our QM/MM protocol for Ps, providing a framework that can be systematically extended to more complex solvents and biological environments, thereby advancing the theoretical interpretation of Ps annihilation in complex systems.

A novel, positron-driven chemical bond is shown to fundamentally stabilize the Be ₂ dimer through a unique two-stage mechanism of e ⁺ redistribution.

Surface hopping simulations critically depend on the accuracy and robustness of the underlying electronic structure methods. Fully correlated approaches—such as CASPT2, MRCI, L-PDFT, and MRSF-TDDFT—that account for both dynamic and static electron correlation offer significant promise. Still, their practical application in dynamics remains limited by computational cost and technical challenges. In this perspective, we examine the current status of such methods by analyzing representative surface hopping simulations of fulvene and pyrrole, two prototypical systems for photophysical and photochemical processes. These examples demonstrate that while fully correlated methods improve the description of bond rearrangements and hot ground-state dynamics, partially correlated approaches—such as ADC(2) and TDDFT—remain sufficient for photophysical excited-state relaxation. Across methods, persistent limitations, such as active space instabilities and potential energy discontinuities, imply the need for improved approaches. We argue that expanding the use of generalized active spaces in the short term and advancing large active space algorithms in the long term will be crucial for making high-accuracy nonadiabatic simulations broadly reliable and accessible.

Aqueous-processable materials are desired to produce and commercialize eco-friendly organic solar cells. Despite the achievement of developing aqueous soluble electron donor and acceptor polymers by incorporating polar side chains (SCs), the efficiency of the greenest devices is lower than that of state-of-the-art technology processed on halogenated solvents. To investigate the impact of different substituents on structural and optical properties in solution, we considered the backbone of the PTQ10 polymer with alkyl and alkoxy SCs. We simulated oligomer chains at low and high concentration conditions via classical molecular dynamics simulations, considering both a water/ethanol mixture and chloroform as solvents. Combining an unsupervised machine learning technique and density functional theory calculations, we validated the system size for quantum calculations and investigated the impact of SCs on the excited states. Then, following the sequential QM/MM approach, we determined the absorption spectra of each polymer. From the simulations at high concentrations, we observed the stacking of different oligomers, suggesting that polymer chains already showed aggregation in solution. This is consistent with our experimental findings, as we measured a red shift of the PTQ(8bO2) spectrum when changing from a chloroform mixture to an aqueous mixture. Finally, we investigated idealized dimer interface models, whose presence of electron-donating and electron-accepting groups results in mixed signatures in the absorption spectra, widening our understanding of polymer aggregation.

This study explores the energetic stability and physical properties of 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 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 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 have shorter bond lengths, higher force constants, and higher bond energies.