Component-wise AO Basis Reduction: Norm Loss, Negative Contribution Normalization, and Functional Implications

Atomic orbital (AO) normalization is a foundational assumption in electronic structure theory, yet in practice, the norm of contracted basis functions can deviate from unity due to internal reduction and transformation mechanisms applied by quantum chemistry packages. This work presents a systematic framework for analyzing the physical and numerical consequences of primitive basis function elimination and AO-level norm inconsistency. The implemented methodology quantifies norm loss, separates constructive and destructive contributions, and enables precise renormalization by retaining both positive and negative terms within AO representations. Using two representative systems—a Raman-active carotenoid (lycopene) and a phosphorus dimer with through-space J(P–P) coupling—sensitivity to AO normalization was evaluated. While vibrational frequencies remained stable across normalization schemes, Raman intensities and J-coupling constants showed non-negligible shifts: up to 6 Hz for phosphorus and over 50 units in Raman activity. The results demonstrate that AO normalization is not merely a numerical refinement, but a physically impactful step with implications for precision spectroscopy and quantum computing applications.