A General Accuracy Ladder for Molecular Structures: From Bonds to Interaction-Driven, Transferable Building Blocks

Structural interpretation of accurate experimental data becomes increasingly challenging when nonlocal effects become important, particularly in delocalized open-shell systems and flexible hydrogen-bonded molecules. In these regimes, density functional methods, even when augmented by bond corrections, become insufficient because the transferable unit of the structural error is defined at an inappropriate scale. Here, we show that transferable corrections can instead be formulated by shifting from individual bonds to larger interaction-driven building blocks. These can be identified automatically and assigned, in a black-box fashion, to the appropriate rung of a general accuracy ladder, thereby establishing a direct correspondence between the scale at which errors arise and the level of electronic-structure theory required to describe them, while retaining an affordable computational cost. Across radicals, nonplanar aromatic systems, and flexible hydrogen-bonded networks, quantitative agreement between theory and experiment is recovered only when the physically relevant transferable unit of the error is matched to a commensurate level of theory and vibrational averaging is treated consistently. Accurate structures are therefore obtained not by uniformly increasing the level of theory but by matching the scale of error transferability with the appropriate level of description. Within this framework, multilevel strategies naturally emerge for large systems in which different regions are treated at different rungs of the ladder according to their structural complexity. This enables available experimental rotational constants to be reproduced within ∼0.1-0.2% at affordable cost, corresponding to root-mean-square deviations of atomic positions on the order of 1-2 × 10^-3 Å.