Spin-strain coupling of a neutral carbon-dimer triplet in monolayer hBN

Optically addressable spin defects make atomically thin hexagonal boron nitride (hBN) promising for quantum sensing, yet published hBN spin-strain coefficients have focused on the negatively charged boron vacancy. Here we calculate dipolar zero-field splitting and spin-strain coupling for a chemically distinct, neutral same-sublattice carbon dimer, CN-CN-3, whose S = 1 ground state has been predicted. PBE-D3(BJ) scans and fixed-geometry HSE06 endpoint calculations probe biaxial, armchair and zigzag deformation over ±2%. The resolved responses are linear and strongly anisotropic. Armchair strain tunes the axial parameter D at +29.2 MHz per %, while its effect on E is 4.2 times weaker. Zigzag strain leaves D unchanged within a <2 MHz per % bound and tunes the signed rhombic parameter E at -28.6 MHz per %, driving it through zero. Computed points bracket E = 0 near +1% tension in the PBE production protocol (moving toward zero with larger cells) and at 2.4-3.0% compression with HSE06. The unstrained PBE splitting is D = 446 MHz and E = 36 MHz. Compared under matched loading, the boron vacancy has substantially larger axial coupling, whereas its rhombic response is comparable. The dimer's distinctive feature is nearly separate mechanical control of D and E in a charge-neutral, vacancy-free defect.