examine the reliability of the CCSD(T) results, as this state exhibits strong electron-correlation effects. The results of this analysis are outlined in Table of the Supplementary Information (SI). Harmonic frequency calculations are performed by means of numerical differentiation of analytically evaluated gradients using the analytic-gradient techniques for the CC methods and the SFX2C-1e scheme as implemented in the CFOUR program package. FC simulations presented here employ the doubleharmonic approximation using the fcsquared module of the CFOUR program.