C2NP: A Benchmark for Learning Scale-Dependent Geometric Invariances in 3D Materials Generation

TL;DR

This paper introduces C2NP, a benchmark for evaluating how well generative models can learn geometric invariances when transitioning from infinite crystalline structures to finite nanoparticles, highlighting current limitations in generalization.

Contribution

The paper presents a new benchmark dataset and tasks for assessing the generalization of generative models across scale-dependent geometric invariances in 3D materials.

Findings

Models often fail to generalize under distribution shift despite low loss.

Current methods tend to memorize templates rather than learn scalable physical principles.

C2NP provides a systematic framework for diagnosing geometric generalization failures.

Abstract

Generative models for materials have achieved strong performance on periodic bulk crystals, yet their ability to generalize across scale transitions to finite nanostructures remains largely untested. We introduce Crystal-to-Nanoparticle (C2NP), a systematic benchmark for evaluating generative models when moving between infinite crystalline unit cells and finite nanoparticles, where surface effects and size-dependent distortions dominate. C2NP defines two complementary tasks: (i) generating nanoparticles of specified radii from periodic unit cells, testing whether models capture surface truncation and geometric constraints; and (ii) recovering bulk lattice parameters and space-group symmetry from finite particle configurations, assessing whether models can infer underlying crystallographic order despite surface perturbations. Using diverse materials as a structurally consistent testbed, we construct over 170,000 nanoparticle configurations by carving particles from supercells derived from DFT-relaxed crystal unit cells, and introduce size-based splits that separate interpolation from extrapolation regimes. Experiments with state-of-the-art approaches, including diffusion, flow-matching, and variational models, show that even when losses are low, models often fail geometrically under distribution shift, yielding large lattice-recovery errors and near-zero joint accuracy on structure and symmetry. Overall, our results suggest that current methods rely on template memorization rather than scalable physical generalization. C2NP offers a controlled, reproducible framework for diagnosing these failures, with immediate applications to nanoparticle catalyst design, nanostructured hydrides for hydrogen storage, and materials discovery. Dataset and code are available at https://github.com/KurbanIntelligenceLab/C2NP.