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

Abstract

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.