The spectrum was computed using the xsim module in CFOUR, with the basis of 50 harmonic states in each mode and 6,000 iterations of the Lanczos procedure, starting from the seed vector with no vibrational excitations, i.e, |seed = |0 . . . 0 . 44 All electrons were correlated in all CC/EOM-CC calculations. All calculations were performed using CFOUR.