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Simon Müller

Publications and source records attributed to Simon Müller.

At least 19 recordsLinked to original sources

An Electrolyte Equation of State Based on openCOSMO-RS: From Aqueous Solutions to Non-Aqueous Systems

Predictive COSMO-RS-based electrolyte models such as COSMO-RS-ES describe electrolyte solutions from quantum-chemical information, but treat the liquid as incompressible and give no access to densities or pressure effects. This work presents openCOSMO-RS-ES-Phi, to our knowledge the first open-source implementation of a predictive electrolyte equation of state based on openCOSMO-RS. It combines the COSMO-RS-based equation of state openCOSMO-RS-Phi with the ion interaction energies and the long-range term of COSMO-RS-ES, which is first refitted on the same simplified short range. With this refit, a dielectric decrement improves the mean ionic activity coefficients (MIAC), whereas a Born term counts the solvation of the ions a second time and is therefore left out. The equation of state then adds only volume, dispersion and temperature parameters of the ions, fitted to aqueous MIAC and densities. openCOSMO-RS-ES-Phi reproduces COSMO-RS-ES in dilute aqueous solutions, improves on it in concentrated ones and also yields solution densities. Extrapolating from aqueous systems, it predicts the MIAC in water + cosolvent mixtures as accurately as the fitted COSMO-RS-ES, as well as the MIAC and bubble points of a benchmark database of water-alcohol-salt systems. Salting-out of organic components is not described as accurately, and salts come out too soluble in organic solvents. Overall, the interaction model developed for COSMO-RS-ES carries over to a large extent to an equation of state, which makes openCOSMO-RS-ES-Phi a sound base model for further development, with ion transfer between solvents as a clear target for fitting to non-aqueous data.

physics.chem-ph↗

Conformer diversity over quantity in COSMO-RS-based LLE predictions

Accurate prediction of phase equilibria is essential for chemical process design, particularly in systems characterized by complex intermolecular interactions. This work investigates the influence of conformer selection and sigma-profile characteristics on COSMO-RS predictions of liquid-liquid equilibria (LLE), using the tetrahydrofurfuryl alcohol (THFA)-methyl isobutyl ketone (MIBK)-water system as a case study. Calculations were performed at the TZVP and TZVPD-FINE levels of theory. While TZVP failed to capture the experimentally observed phase behavior, TZVPD-FINE provided substantially improved predictions. The differences are attributed to the relative importance assigned to less-polar conformers and more-polar conformers capable of hydrogen bonding. Although the TZVPD-FINE calculations reproduced the main experimental trends, the predicted organic-phase composition showed a mean percentage deviation of 5.27%, slightly exceeding the adopted 5% agreement threshold. Investigations using individual conformers demonstrated that predictions can be strongly influenced by conformer selection and that representative conformer ensembles are necessary for reliable results. Further analysis showed that diversity in sigma profiles is more important for predictive accuracy than the total number of conformers. These findings emphasize the importance of carefully selecting conformers when applying COSMO-RS to liquid-liquid equilibria involving phases of contrasting polarity.

physics.chem-ph↗

CRAFT-ε: A Conformer-Resolved Atom- and Fragment-based Theory for the Static Relative Permittivity of Pure Organic Liquids

The static relative permittivity is an important property in thermodynamic modeling, electrochemistry, and solvent design, but predicting it directly from molecular structure remains challenging because both electronic polarization and molecular dipole orientation contribute to the measured response. Here, we introduce CRAFT-ε (Conformer-Resolved Atom- and Fragment-based Theory for ε), a conformer-resolved molecular descriptor model for predicting the static relative permittivity of neutral pure liquids. Conformer free energies, molecular dipoles, and atom-resolved polarizabilities are obtained from a GFN2-xTB/ALPB 10 workflow. The molecular dipole is decomposed into chemically defined polar-fragment vectors, allowing fitted corrections to retain both the magnitude and relative orientation of local dipolar contributions. Electronic and orientational polarization are combined with a group-contribution molar volume in the Onsager-Kirkwood-Fröhlich relation. A staged parameterization with 41 parameters is applied to nonpolar, polar, and hydrogen-bonding compounds. For 1393 liquids with permittivities between 1.5 and 183, CRAFT-ε reaches a five-fold cross-validated average absolute deviation of 2.40 ({R^2} = 0.93). Conformational averaging 15 is essential for liquids with several polar groups. Compared with an established fragment-based model with 139 parameters, it reaches a similar cross-validated RMSE (3.65 versus 3.41) with about a quarter of the parameters and applies to every liquid of the dataset. The resulting framework provides a computationally inexpensive route from molecular structure to liquid dielectric response and forms a basis for later application to solvent mixtures and electrolyte systems.

physics.chem-ph↗

GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmosphere

HATS-6 b is one of several recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ($\mathrm{T_{eq}}\sim700$ K), Saturn-mass ($M_p\sim0.3~\mathrm{M_J}$), Jupiter-radius ($R_p\sim1~\mathrm{R_J}$) planet that transits its star every $\sim$ 3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of $0.6-5.3$ um. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low metallicity ($\log\mathrm{[M/H]}=-1.99^{+0.2}_{-0.2}$) sub-solar C/O ($\log\mathrm{[C/O]=-0.46^{+0.2}_{-0.2}}$) atmosphere, and find strong evidence for H$_2$O, CH$_4$, and NH$_3$ at volume mixing ratios (in $\log[X]$) of $-4.88_{-0.24}^{+0.25}$, $-5.38_{-0.19}^{+0.18}$, and $-6.03_{-0.19}^{+0.18}$, respectively. We consistently retrieve a significantly lower $\mathrm{T_{eq}}$ than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a non-zero bond albedo. Our planetary interior models retrieve bulk metallicities three orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well-mixed. We find an excess feature around 3 um, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C$_2$H$_4$. Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.

astro-ph.EP↗

Giant exoplanets are not fully mixed

The interior structure and bulk composition of giant planets are not directly observable and must be inferred from models. Under the common assumption of a well-mixed, adiabatic envelope, the measured atmospheric metallicity is taken as a proxy for the metallicity of the entire envelope, and hence for the planet's heavy-element budget. JWST now provides precise atmospheric metallicities for a growing number of warm giants, allowing this assumption to be tested for the first time. We quantify the difference between envelope and bulk metallicities of warm giants to assess the evidence for compositional stratification. We assembled eleven warm giants with atmospheric metallicities from published JWST retrievals, computed tailored interior and thermal evolution model grids for each, and performed MCMC retrievals to infer the bulk metallicity consistent with the measured mass, radius, system age, and atmospheric metallicity. Envelope metallicities are smaller than bulk metallicities throughout the sample, with mixing ratios from about 0.02 to 0.90. Eight of the eleven planets have mixing ratios below 0.50, and ten are inconsistent with a fully mixed interior to within one sigma. We tentatively identify a significant anti-correlation between planetary mass and envelope metallicity, but no correlation between envelope and bulk metallicity, nor between envelope and host-star metallicity. Atmospheric metallicity is therefore not a reliable proxy for the bulk composition of warm giants, and incomplete mixing (possibly composition gradients) appears common among the planets accessible to JWST. Bulk composition estimates assuming a homogeneous envelope substantially underestimate the total heavy-element mass. That the solar-system giants are unremarkable within this sample suggests dilute or partially mixed interiors may be a generic outcome of giant planet formation.

astro-ph.EP↗

Deep radiative zones affect giant planet cooling and internal structure: Implications for exoplanet characterisation

The radiative opacity plays a critical role in shaping the thermal evolution and interior structure of giant planets. Near $\sim$ 2,000 K, a hydrogen-transparency region creates a window of reduced opacity that can give rise to detached, deep radiative zones between two convective zones. This local opacity minimum could be deepened further by alkali depletion. While such zones have been explored for Jupiter and Saturn, their influence on cold to warm giant exoplanets remains unstudied. We investigate how opacity windows and the resulting deep radiative zones affect the cooling, radius evolution, and characterisation of giant exoplanet interiors and atmospheres. We computed thermal evolution models for cold to warm Jupiters spanning masses of 0.3 to 4.0 M$_{\rm{J}}$, envelope metallicities from one to ten times solar, equilibrium temperatures of 100 to 800 K, and a parametrised reduction in the radiative opacity. Detached deep radiative zones develop in moderately irradiated Jupiters older than a few gigayears even with unmodified opacities, and earlier and more extensively when the opacity is reduced. The age and equilibrium temperature at which they appear depend on planetary mass, envelope metallicity, and opacity, with metal enrichment suppressing them at low equilibrium temperatures but promoting them at higher ones. A deep opacity window accelerates cooling, reducing predicted radii by up to 5% and interior temperatures by several tens of percent, which translates to a $\sim$ 10 percentage point difference in the inferred bulk metallicity. Deep radiative zones are likely common in warm giant exoplanets and could decouple atmospheric composition from bulk interior composition, complicating the interpretation of atmospheric observations. The opacity treatment therefore introduces significant uncertainties in atmospheric and interior characterisation.

astro-ph.EP↗

Representations from Pretrained Machine-Learning Interatomic Potentials as Coarse Coordinates for Material Generation and Evaluation

Generative machine learning is increasingly used for inorganic crystal structure generation. Most models and the corresponding evaluation approaches rely on simple forms of crystal structure representation. In this paper, we showcase the power of atom-averaged features from pretrained Machine-Learning Interatomic Potentials (MLIPs), such as MACE, for such tasks. We first introduce a distance measure that assesses the output of material generative models by capturing both quality and novelty in a single distribution-based evaluation framework. In particular, we introduce the Coarse-Fine Transport Distance (CFTD) using two different featurizers, where the quality component is based on coarse MACE features. We showcase CFTD's versatility in capturing crystal-structure quality while also detecting memorization, and compare it with the recently introduced continuous SUN metrics. We further show that coarse MACE features can be used as guidance for a material generative model.

cs.LG↗

Extension of openCOSMO-RS Into a Full Open-Source Equation of State: Implementation, Parameterization, and Benchmarking

The COSMO-SAC-Phi model developed by Soares et al. extends the COSMO-SAC activity-coefficient framework into a full equation of state by explicitly accounting for pressure effects. In this approach, pure substances and mixtures are represented as pseudo-mixtures consisting of the actual number of moles and an additional pseudo-component that describes free volume, or holes. In this work, we implement this extension within the openCOSMO-RS framework and evaluate it using a large and diverse set of molecules and binary systems. The resulting equation of state includes an extensive open-source parameter set with around 1800 pure-component entries, made freely available to the academic community. The four pure-component parameters were fitted to vapor-pressure and liquid-molar volume data for each substance. Model performance was assessed against two benchmark equation-of-state databases, one for pure compounds and one for binary mixtures, without introducing any binary interaction parameters. The resulting openCOSMO-RS-Phi model reproduces the accuracy of the original COSMO-SAC-Phi formulation while providing a fully open-source and accessible implementation for the scientific community. Beyond its immediate utility, it also establishes a foundation for future development of predictive EoS for electrolyte solutions.

physics.chem-ph↗

Resizing the giants: How modelling adiabatic interiors impacts predicted planetary radii

The interiors of giant planets are commonly assumed to be convective and adiabatic, making the adiabatic temperature gradient a key ingredient in interior and evolution models. Multiple numerically distinct methods exist for computing this gradient, yet their impact on inferred planetary structure and radius has not been systematically assessed. In this letter we investigate how the numerical treatment of adiabatic temperature profiles affects inferred planetary radii and internal structure, comparing different methods for evaluating the adiabatic gradient against a ground-truth isentropic baseline, for both the logarithmic and non-logarithmic forms of the temperature differential equation. Static interior models of a one Jupiter mass planet were computed using a state-of-the-art hydrogen-helium equation of state. We find that the choice of numerical method significantly impacts the inferred interior structure and radius. Using the logarithmic temperature equation, central temperatures deviate by several thousand kelvin and surface radii differ by up to 3.4 per cent, exceeding the 1 per cent precision of current giant exoplanet radius measurements threefold. The non-logarithmic form reduces deviations to below ~1 per cent for most methods. We recommend spline derivatives to evaluate the adiabatic gradient, combined with the non-logarithmic temperature equation. Finite differencing and direct use of tabulated gradients or derivatives should be avoided.

astro-ph.EP↗

Origin and characterization of super-Earths and sub-Neptunes

Super-Earths and sub-Neptunes represent the most common class of exoplanets discovered to date in our galaxy, yet they have no direct analogues in the Solar System. Since 2014, researchers within the NCCR PlanetS have made significant contributions to understanding the origin and nature of these small planets. This chapter provides an overview of the progress made in their detection, characterization, and theoretical interpretation during the 2014-2025 period. The combined data from space-based photometric missions such as Kepler and TESS, together with ground-based radial velocity campaigns using state-of-the-art spectrographs (e.g., HARPS, ESPRESSO, NIRPS), have enabled detailed demographic analyses of these planets. These observational efforts are complemented by theoretical work exploring their internal structures, bulk compositions, formation and evolution, shedding light on the physical processes responsible for the observed diversity. As high-precision observations from facilities like JWST begin to probe the atmospheric composition of individual planets, a more complete picture of super-Earth and sub-Neptune origins is emerging, one that continues to challenge and refine current planet formation theories.

astro-ph.EP↗

Long-period transiting exoplanets: advances in detection and characterization

Most detected transiting planets have orbits which would fit within the one of Mercury, exposing them to intense stellar irradiation and interactions that significantly alter their properties. In contrast, colder planets with longer orbital periods are less affected, offering crucial insights into their formation and migration histories. Characterizing transiting warm and temperate planets is a key missing piece in the exoplanet puzzle. Dedicated photometric and spectroscopic follow-up of transiting events detected in space-based photometric data opened the way to detecting long-period transiting exoplanets. The wealth of information available for these transiting planets makes them golden targets for in-depth characterization. For giant planets, combining precise masses, radii, and ages with state-of-the-art planetary evolution models allows the estimation of their planetary bulk compositions, a crucial element to explore their formation and evolution pathways. Furthermore, these planets are compelling candidates for hosting moons and circumplanetary rings-features that could illuminate dynamical histories, satellite formation processes, and even potential habitable environments.

astro-ph.EP↗

GEMS JWST: HATS-75 b -- A giant planet with a sub-solar metallicity atmosphere orbiting an M-dwarf

HATS-75 b is one of the recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying 7 GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6-5.3 um). The planet's spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (.e.g, stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity / TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity (log[M/H]=-1.74^{+0.92}_{-0.76}) and super-solar carbon-to-oxygen (C/O=1.04^{+0.40}_{-0.09}), which paired with a best-fit interior model with bulk metallicity of Z_p=0.20+/-0.04, implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH4, CO, and CO2. We obtain only an upper limit for H2O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M-dwarfs with JWST.

astro-ph.EP↗

GEMS JWST: A sub-Solar metallicity atmosphere for giant planet TOI-5293Ab orbiting a rapidly changing M-dwarf

The growing sample of Giant Exoplanets around M-dwarf Stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293Ab, a 0.5 $M_J$ planet orbiting an early M-dwarf with a period of $\sim$ 3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope ${<1}$ $μ$m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ($\log [\mathrm{M/H}] = -1.03^{+0.53}_{-0.44}$ $\times$ Solar) and slightly super-solar C/O ratio ($1.23^{+2.94}_{-0.75}$). The retrievals yield Bayes factors that indicate strong evidence for \ce{CH4} as well as low significance detections of \ce{CO2}, \ce{H2O}, \ce{NH3}. Finally, using thermal evolution models we find that the radius of TOI-5293Ab is inflated above theoretical expectations ($\sim$ 1.07 $R_J$), despite it having an temperature of $\sim$ 700 K, and hence we were unable to constrain its bulk composition.

astro-ph.EP↗

GEMS JWST: Transmission spectroscopy of TOI-5205b reveals significant stellar contamination and a metal-poor atmosphere

Recent discoveries of transiting giant exoplanets ($R_p\gtrsim8\mathrm{~R_\oplus}$) around M dwarfs (GEMS) present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ($P=1.63~\mathrm{days}$) Jupiter-like planet ($M_p=1.08~\mathrm{M_J}$ and $R_p=0.94~\mathrm{R_J}$) orbiting an M4 dwarf ($M_\star=0.392~\mathrm{M_\odot}$, $R_\star=0.394~\mathrm{R_\odot}$). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph (NIRSpec) spanning $0.6-5.3$ um. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths $λ\lesssim3.0$ um, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of $\mathrm{H_2O}$. The wavelength coverage enabled a robust detection of $\mathrm{CH_4}$ and $\mathrm{H_2S}$, which have detectable molecular features between $3.0-5.0$ um. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with sub-solar metallicity ($3σ$ upper limit of $\log\mathrm{[M/H]}\lesssim-1.24$) and super-solar C/O ratio ($3σ$ lower limit of $\log\mathrm{[C/O]}\gtrsim0.09$), although this may partly be driven by the non-detection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10--20\%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

astro-ph.EP↗

Transport Novelty Distance: A Distributional Metric for Evaluating Material Generative Models

Recent advances in generative machine learning have opened new possibilities for the discovery and design of novel materials. However, as these models become more sophisticated, the need for rigorous and meaningful evaluation metrics has grown. Existing evaluation approaches often fail to capture both the quality and novelty of generated structures, limiting our ability to assess true generative performance. In this paper, we introduce the Transport Novelty Distance (TNovD) to judge generative models used for materials discovery jointly by the quality and novelty of the generated materials. Based on ideas from Optimal Transport theory, TNovD uses a coupling between the features of the training and generated sets, which is refined into a quality and memorization regime by a threshold. The features are generated from crystal structures using a graph neural network that is trained to distinguish between materials, their augmented counterparts, and differently sized supercells using contrastive learning. We evaluate our proposed metric on typical toy experiments relevant for crystal structure prediction, including memorization, noise injection and lattice deformations. Additionally, we validate the TNovD on the MP20 validation set and the WBM substitution dataset, demonstrating that it is capable of detecting both memorized and low-quality material data. We also benchmark the performance of several popular material generative models. While introduced for materials, our TNovD framework is domain-agnostic and can be adapted for other areas, such as images and molecules.

cond-mat.mtrl-sci↗

Giant Planet Evolution with MESA

The evolution of gaseous planets is a complex process influenced by various physical parameters and processes. In this study, we present critical modifications to the Modules for Experiments in Stellar Astrophysics (MESA) code to enhance its applicability to giant planet modelling. We introduce an equation of state specifically tailored for materials at planetary conditions. The equation of state considers the thermodynamic properties of hydrogen-helium mixtures and heavy elements, improving the accuracy of internal structure calculations. We also present modifications to the radiative opacity to allow the modelling of grains, clouds and opacity windows. Furthermore, we refine the treatment of convective mixing processes in MESA to better replicate convective mixing with the presence of composition gradients. Finally, we add a treatment for helium rain and settling. These modifications aim to enhance the predictive capabilities of MESA for giant planet evolution and are publicly available. We hope that these improvements will lead to a deeper understanding of giant planet evolution in the solar system and beyond.

astro-ph.EP↗

Towards Effective Complementary Security Analysis using Large Language Models

A key challenge in security analysis is the manual evaluation of potential security weaknesses generated by static application security testing (SAST) tools. Numerous false positives (FPs) in these reports reduce the effectiveness of security analysis. We propose using Large Language Models (LLMs) to improve the assessment of SAST findings. We investigate the ability of LLMs to reduce FPs while trying to maintain a perfect true positive rate, using datasets extracted from the OWASP Benchmark (v1.2) and a real-world software project. Our results indicate that advanced prompting techniques, such as Chain-of-Thought and Self-Consistency, substantially improve FP detection. Notably, some LLMs identified approximately 62.5% of FPs in the OWASP Benchmark dataset without missing genuine weaknesses. Combining detections from different LLMs would increase this FP detection to approximately 78.9%. Additionally, we demonstrate our approach's generalizability using a real-world dataset covering five SAST tools, three programming languages, and infrastructure files. The best LLM detected 33.85% of all FPs without missing genuine weaknesses, while combining detections from different LLMs would increase this detection to 38.46%. Our findings highlight the potential of LLMs to complement traditional SAST tools, enhancing automation and reducing resources spent addressing false alarms.

cs.CR↗

On the origin of Jupiter's fuzzy core: constraints from N-body, impact and evolution simulations

It has been suggested that Jupiter's fuzzy core could be a result of a giant impact. Here, we investigate the expected impact conditions from N-body simulations. We then use state-of-the-art SPH simulations to investigate the results of impacts with different conditions including various impactor masses and composition, different formation stages in Jupiter's growth, and different resolutions. We next simulate the long-term thermal evolution of Jupiter post-impact. We find that 3D N-body simulations predict rather oblique impacts, and that head-on collisions are rare. Moreover, our results show that even under a head-on collision, Jupiter's fuzzy core cannot be formed. We next simulated Jupiter's thermal evolution and showed that unless post-impact temperatures are extremely low, a giant impact would not lead to an extended dilute core as inferred by interior models. We conclude that Jupiter's fuzzy core is not caused by an impact and is likely to be an outcome of its formation process.

astro-ph.EP↗