Supplemental Material

Normal modes of formaldehyde (Table S1). The adiabatic effective q4 potential derived from the lowest two spectator-mode vibrational eigenenergies along the tunneling coordinate (Figure S1). Term energies and residuals of the A-X 1^1 and 4^1 5^1 levels (Tables S2, S3). Comparison of EOM-CCSDT/ANO1 variational calculations with experimental band origins and shift in rotational constants (Table S4). Simulated IR Q-branch for the K'_a=1 manifold, an IR-UV double-resonance LIF spectrum, and a LIF time trace, associated with the 1^1 level (Figures S2, S3, S6). Reduced term value plots and energy level residuals for the A 1^1 and 4^1 5^1 rovibrational levels (Figures S4, S6).