arXiv subjects

Aneesa Ahmad

Publications and source records attributed to Aneesa Ahmad.

3 recordsLinked to original sources

arXiv2026

Binding Energies of Astrophysically Relevant Molecules on CO$_2$ Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding

Carbon dioxide (CO$_2$) is among the most abundant interstellar ices and has been observed by JWST in water-rich mixtures and pure CO$_2$ environments. Astrochemical models adopt binding energies (BEs) measured on water ice because few BEs are available for CO$_2$ ice. We calculate BE distributions for twelve adsorbates (N$_2$, CH$_4$, HCl, C$_2$H$_2$, H$_2$S, HF, HCN, H$_2$CO, HCOOH, HNC, CH$_3$OH, H$_2$O) on CO$_2$ clusters containing two to five molecules using DFT calibrated against CCSD(T)/CBS, a coupled-cluster treatment extrapolated to the complete-basis-set limit. Binding strength depends on whether the adsorbate requires the surface to donate a hydrogen bond (H-bond). CO$_2$ can accept an H-bond but cannot donate one, and all six species compared with water ice from Ahmad et al. (2026) bind more weakly on CO$_2$. The ratio of the BE on CO$_2$ to water ice ranges from 0.66 for H$_2$CO and 0.75 for H$_2$O to 0.87 for CH$_3$OH, 0.89 for HCOOH and 0.92 for C$_2$H$_2$. Formaldehyde is predicted to desorb near 57 K from CO$_2$-rich ice, compared with 85 K from water-rich ice, and could contribute to cold gas-phase H$_2$CO. Although the mean binding energies are changing with cluster size for several species, the differences can be traced to local binding interactions likely to remain important in larger ice models. The BEs vary by more than 500 K between sites, while the desorption peaks range from 11 K for CH$_4$ to 132 K for HCOOH. The limited H-bonding capacity of CO$_2$ may favour separation of mixed H$_2$O:CO$_2$ ice into CO$_2$-rich domains.

astro-ph.GA↗

arXiv2026

AstroBind: Machine learning prediction of binding energy distributions on interstellar water ice from geometric surface descriptors

Binding energies on amorphous solid water regulate molecular desorption in molecular clouds and protoplanetary disks, but computing them from first principles across the heterogeneous binding sites of disordered ice surfaces is computationally prohibitive. We aim to develop a rapid, interpretable method for predicting binding energies on amorphous solid water that captures surface heterogeneity without requiring explicit electronic-structure calculations. We trained a machine-learning model using 27 geometric descriptors of the local ice-adsorbate environment. The model was trained on binding energies for 13 adsorbates on amorphous solid water clusters and evaluated through pooled leave-one-cluster-out validation across 15 ice surfaces. Its transferability was assessed on radical species excluded entirely from training. The model achieves a pooled leave-one-cluster-out coefficient of determination $R^2 = 0.90$ (the fraction of variance in the binding energies captured by the model, where 1 is a perfect fit), and a mean absolute error of 378 K across 15 ice surfaces. It also transfers to radical species withheld from training, with a pooled $R^2 = 0.79$, although its accuracy decreases when the unpaired electron contributes directly to the surface interaction. Our results demonstrate a route towards rapid, distribution-aware parameterisation of binding energies and desorption/diffusion rates in gas-grain astrochemical models, while preserving physical interpretability on heterogeneous ice surfaces.

astro-ph.GA↗

arXiv2026

Theoretical determination of the binding energies of methanol and related species onto amorphous solid water ice

The formation and survival of complex organic molecules (COMs) in cold interstellar environments depends on their interactions with icy dust grain surfaces. Methanol, a key COM detected in cold cores and protoplanetary disks, is believed to form on amorphous solid water (ASW) through surface reactions and reside there until it is desorbed into the gas phase. We present a theoretical study of the binding energies (BEs) of methanol and its photolysis-derived species on ASW clusters by means of dispersion-corrected density functional theory (DFT) using a refined protocol implemented in the Binding Energy Evaluation Platform (BEEP). Molecules capable of hydrogen bonding, such as H2O, CH3OH, HCOOH, and OH, exhibit high BEs and broad BE distributions that reflect the structural heterogeneity of the ASW surface. In contrast, weakly interacting volatiles including CO, CO2, CH4, and CH3 display narrower distributions dominated by dispersion interactions. Open-shell radicals such as CH2OH and OH bind more strongly than HCO and CH3 due to their ability to form directional hydrogen bonds. Incorporation of our BEs into an astrochemical model, in conjunction with a recalculation of the pre-exponential factor using transition state theory, demonstrates the sensitivity of model results to the method of calculation of the grain-surface reaction rates. The new approach generally predicts a higher abundance of radicals on the ice that are key reactants for the formation of COMs when surface diffusion is assumed to be efficient. These findings emphasize the importance of incorporating BEs that have been determined in a self-consistent manner into astrochemical models, and provide reliable theoretical benchmarks for species with limited experimental data.

astro-ph.GA↗