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1. Modeling glasses from first principles using random structure sampling

   https://doi.org/10.1063/5.0243888  

   Wolf, Laszlo; Novick, Andrew; Stevanović, Vladan (March 2025, Journal of Applied Physics)

   We present an approach to approximating static properties of glasses without experimental inputs rooted in the first-principles random structure sampling. In our approach, the glassy system is represented by a collection (composite) of periodic, small-cell (few 10 s of atoms) local minima on the potential energy surface. These are obtained by generating a set of periodic structures with random lattice parameters and random atomic positions, which are then relaxed to their closest local minima on the potential energy surface using the first-principles methods. Using vitreous SiO2 as an example, we illustrate and discuss how well various atomic and electronic structure properties calculated as averages over the set of such local minima reproduce experimental data. The practical benefit of our approach, which can be rigorously thought of as representing an infinitely quickly quenched liquid, is in that it transfers the computational burden to linear scaling and easy to converge averages of properties computed on small-cell structures, rather than simulation cells with 100 s if not 1000 s of atoms while retaining a good overall predictive accuracy. Because of this, it enables the future use of high-cost/high-accuracy electronic structure methods, thereby bringing the modeling of glasses and amorphous phases closer to the state of modeling of crystalline solids. 

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2. Direct free energy evaluation of classical and quantum many-body systems via field-theoretic simulation

   https://doi.org/10.1073/pnas.2201804119  

   Fredrickson, Glenn H.; Delaney, Kris T. (May 2022, Proceedings of the National Academy of Sciences)

   Free energy evaluation in molecular simulations of both classical and quantum systems is computationally intensive and requires sophisticated algorithms. This is because free energy depends on the volume of accessible phase space, a quantity that is inextricably linked to the integration measure in a coordinate representation of a many-body problem. In contrast, the same problem expressed as a field theory (auxiliary field or coherent states) isolates the particle number as a simple parameter in the Hamiltonian or action functional and enables the identification of a chemical potential field operator. We show that this feature leads a “direct” method of free energy evaluation, in which a particle model is converted to a field theory and appropriate field operators are averaged using a field-theoretic simulation conducted with complex Langevin sampling. These averages provide an immediate estimate of the Helmholtz free energy in the canonical ensemble and the entropy in the microcanonical ensemble. The method is illustrated for a classical polymer solution, a block copolymer melt exhibiting liquid crystalline and solid mesophases, and a quantum fluid of interacting bosons. 

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3. Calculation of rovibrational eigenstates of H3+ using ScalIT

   https://doi.org/10.1063/5.0047823  

   Sarka, János; Das, Debojyoti; Poirier, Bill (April 2021, AIP Advances)

   H 3 + is a key player in molecular astrophysics, appearing in the interstellar medium and in the atmospheres of gas giants. It also plays an important role in star formation, and it has also been detected in supernova remnants. In theoretical chemistry, H3+ has long been a benchmark polyatomic system for high-level electronic-structure computations, as well as for quantum dynamics studies. In this work, exact quantum dynamical calculations are carried out for H3+, using the ScalIT suite of parallel codes, applied to two spectroscopically accurate potential energy surfaces. Specifically, rovibrational energy levels and wavefunctions are computed and labeled. Sixty vibrational states (for J = 0) are first determined, and then, rotational excitations for each of these “vibrational parent” states are computed up to total angular momentum J = 46, which is the highest value for which bound states of this molecule exist (D0 ∼ 35 000 cm−1). For these calculations, a very tight basis set convergence of a few 10−4 cm−1 (or less) has been achieved for almost all the computed energy levels. Where comparisons can be made, our results are found to agree well with earlier calculations and experimental data. 

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4. Comparing parameterized and self-consistent approaches to ab initio cavity quantum electrodynamics for electronic strong coupling

   https://doi.org/10.1063/5.0230565  

   Manderna, Ruby; Vu, Nam; Foley, IV, Jonathan_J (November 2024, The Journal of Chemical Physics)

   Molecules under strong or ultra-strong light–matter coupling present an intriguing route to modify chemical structure, properties, and reactivity. A rigorous theoretical treatment of such systems requires handling matter and photon degrees of freedom on an equal quantum mechanical footing. In the regime of molecular electronic strong or ultra-strong coupling to one or a few molecules, it is desirable to treat the molecular electronic degrees of freedom using the tools of ab initio quantum chemistry, yielding an approach referred to as ab initio cavity quantum electrodynamics (ai-QED), where the photon degrees of freedom are treated at the level of cavity QED. We analyze two complementary approaches to ai-QED: (1) a parameterized ai-QED, a two-step approach where the matter degrees of freedom are computed using existing electronic structure theories, enabling the construction of rigorous ai-QED Hamiltonians in a basis of many-electron eigenstates, and (2) self-consistent ai-QED, a one-step approach where electronic structure methods are generalized to include coupling between electronic and photon degrees of freedom. Although these approaches are equivalent in their exact limits, we identify a disparity between the projection of the two-body dipole self-energy operator that appears in the parameterized approach and its exact counterpart in the self-consistent approach. We provide a theoretical argument that this disparity resolves only under the limit of a complete orbital basis and a complete many-electron basis for the projection. We present numerical results highlighting this disparity and its resolution in a particularly simple molecular system of helium hydride cation, where it is possible to approach these two complete basis limits simultaneously. In this same helium hydride system, we examine and compare the practical issue of the computational cost required to converge each approach toward the complete orbital and many-electron bases limit. Finally, we assess the aspect of photonic convergence for polar and charged species, finding comparable behavior between parameterized and self-consistent approaches. 

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5. Improved Characterization of Ultralow‐Velocity Zones Through Advances in Bayesian Inversion of ScP Waveforms

   https://doi.org/10.1029/2023JB026415  

   Pachhai, Surya; Thorne, Michael S.; Rost, Sebastian (June 2023, Journal of Geophysical Research: Solid Earth)

   Abstract Ultralow‐velocity zones (ULVZs) have been studied using a variety of seismic phases; however, their physical origin is still poorly understood. Short period ScP waveforms are extensively used to infer ULVZ properties because they may be sensitive to all ULVZ elastic moduli and thickness. However, ScP waveforms are additionally complicated by the effects of path attenuation, coherent noise, and source complexity. To address these complications, we developed a hierarchical Bayesian inversion method that allows us to invert ScP waveforms from multiple events simultaneously and accounts for path attenuation and correlated noise. The inversion method is tested with synthetic predictions which show that the inclusion of attenuation is imperative to recover ULVZ parameters accurately and that the ULVZ thickness and S‐wave velocity decrease are most reliably recovered. Utilizing multiple events simultaneously reduces the effects of coherent noise and source time function complexity, which in turn allows for the inclusion of more data to be used in the analyses. We next applied the method to ScP data recorded in Australia for 291 events that sample the core‐mantle boundary beneath the Coral Sea. Our results indicate, on average, ∼12‐km thick ULVZ with ∼14% reduction in S‐wave velocity across the region, but there is a greater variability in ULVZ properties in the south than that in the north of the sampled region. P‐wave velocity reductions and density perturbations are mostly below 10%. These ScP data show more than one ScP post‐cursor in some areas which may indicate complex 3‐D ULVZ structures. 

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