A unified machine learning framework for ab initio multiscale modeling of liquids

TL;DR

This paper introduces a unified machine learning framework combining MLIPs and neural cDFT to efficiently model liquids across scales from first principles, accurately predicting thermodynamics and phase behavior.

Contribution

It presents a novel integrated approach that bridges microscopic quantum interactions with macroscopic fluid properties using machine learning and neural density functional theory.

Findings

Accurately reproduces bulk equations of state for water and CO2.

Predicts confinement effects on liquid-vapor coexistence.

Captures complex phenomena like Fisher-Widom and Widom lines.

Abstract

Understanding and predicting the behavior of liquid matter across length scales, using only the microscopic interactions encoded in the Schr\"odinger equation, remains a central challenge in the physical sciences. Achieving this goal requires not only an accurate and efficient description of intermolecular forces but also a consistent framework that bridges the micro-, meso-, and macroscales. Here, by combining machine-learned interatomic potentials (MLIPs) with neural classical density functional theory (neural cDFT), we present such a framework. The underlying idea is simple: MLIPs trained on quantum-mechanical energies and forces are used to generate inhomogeneous microscopic density profiles, which in turn serve as the training data for neural cDFT. The resulting ab initio neural cDFT is not only significantly more computationally efficient than molecular simulations, but also provides a conceptually transparent route to the thermodynamics of both homogeneous and inhomogeneous systems. We demonstrate the approach for both water and carbon dioxide using several exchange-correlation functionals. Beyond accurately reproducing bulk equations of state and liquid-vapor phase diagrams, ab initio neural cDFT predicts, from first principles, how confinement modifies liquid-vapor coexistence in water. It also captures complex behavior in supercritical carbon dioxide such as the Fisher-Widom and Widom lines. Ab initio neural cDFT establishes a general first-principles route to multiscale modeling of fluids within a single unified conceptual framework.