Molecular-Scale Spatial Confinement as a Tunable Handle on Nuclear Tunneling: A DFT Study of Co-Encapsulation of D2 and Ne in C60

Abstract

Quantum tunneling through the Coulomb barrier is highly sensitive to interatomic separation, yet the extent to which molecular-scale spatial confinement can influence this nuclear quantum process remains insufficiently explored. Using density functional theory (PBE/def2-SVP and r2SCAN-D4/def2-TZVPP), it was found that coencapsulation of D2 and Ne in C60 compressed the D–D bond lengths relative to free D2. An optimal Ne loading of ten atoms (D2@10Ne@C60) yielded the shortest D–D interatomic distance among all D2–(noble gas) coencapsulating C60 systems, even smaller than that of D2 confined within the smallest possible fullerene C20 (D2@C20). Shortening the D–D interatomic distance in D2 through spatial confinement can significantly increase the quantum tunneling probability and, consequently, the D–D nuclear fusion rate. The D–D bond length in D2@10Ne@C60 is 15% shorter than that of free D2 according to geometry optimization results. The fusion rate of D2@10Ne@C60 was calculated to be 4 orders of magnitude higher than that of free D2, surpassing D2@C20 (10% D–D bond length reduction and three-orders-of-magnitude enhancement in fusion rate relative to free D2) by 1 order of magnitude. These findings establish a quantitative link between molecular structure and nuclear tunneling, highlighting molecular-scale spatial confinement as a tunable handle on a nuclear quantum process and providing a conceptual connection to pycnonuclear physics, as well as a potential target for experimental investigation through D2@10Ne@C60 synthesis. This study also opens a potential new avenue for nuclear fusion energy research distinct from conventional plasma- and laser-driven approaches.