A Cautionary Tale: Failure of the Valence CASSCF to Describe the Hallmark of Hydrogen Bonding

O dimers fail to capture donor bond lengthening and associated redshift of its stretching frequency due to hydrogen bonding. Analysis of the orbitals in the vCAS calculation reveals that one of the active weakly occupied dimer orbitals correlates electrons in the acceptor lone pairs, not the donor bond, which is left without left-right correlation. This has significant implications for subsequent treatments of dynamical correlation that are based on a vCAS reference wave function.