Carbon capture and storage is a vital strategy for mitigating rising atmospheric carbon dioxide, and metal–organic frameworks (MOFs) have gained attention as promising sorbents. Numerous simulations have examined factors governing CO₂ capture in MOFs—such as diffusion in MOF-74 under varying temperatures and process modeling of MOF-5—but most were limited to specific structures or conditions, hindering a systematic understanding of diffusion across diverse MOFs. Conventional computational methods also face constraints: density functional theory mainly provides static energy evaluations, while molecular dynamics relies on fixed force fields with poor transferability and an inability to describe reactive events. To overcome these limitations, this study employs molecular dynamics simulations driven by neural network potentials to evaluate CO₂ diffusivity in 17 types of MOFs. Results reveal significant variation in transport behavior, with zeolitic-imidazolate framework-3 showing the highest diffusivity and MOF-74 the lowest—an approximately 19-fold difference. These findings highlight the capability of neural-network-based molecular dynamics to deliver consistent and quantitative assessments of CO₂ transport in MOFs, providing a reliable framework for the rational design of next-generation capture materials.

The mole, symbol mol, is the International System of Units (SI) unit of the amount of substance which is the quantity referring to a measure of the number of specified elementary entities, such as chemical elements or compounds in a sample. In the current SI, the mole is defined by specifying the mass of carbon-12. But this base unit is not an invariant of nature because the mass is defined by the material artefact. According to efforts to define the base units in SI using true invariants of nature, the mole will be redefined by fixing the numerical value of a fundamental constant, the Avogadro constant. In the new SI, the definition of the mole can be realized through the experiments that lead to the determination of the Avogadro constant. The best experimental value of the Avogadro constant has been obtained by the X-ray crystal density experiment using silicon-28 highly enriched silicon sphere in the frame work of the International Avogadro Coordination. In this paper, the current definition of the mole and practical aspects of this unit are introduced, then the principle and technical challenges in X-ray crystal density experiment for redefinition of the mole are discussed.