The strongest bond in the universe? Accurate calculation of compliance matrices for the ions N2H+, HCO+, and HOC+

Compliance matrices of protonated CO and N2 are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 Å/mdyn) to CO–H+ the C–O bond is weakened (0.062 Å/mdyn), while the C–O bond in H–CO+ is getting stronger (0.045 Å/mdyn). After protonation, the N–N bond strength is getting stronger (from 0.043 to 0.042 Å/mdyn), too. The invariance of compliance matrix elements Cij under completion of (xi,xj) to a complete set (…,xi,…,xj,…) of internal coordinates is demonstrated.