  An effective Hamiltonian is considered for hydrogen bonding between two
molecules due to the quantum mechanical interaction between the orbitals of the
H-atom and the donor and acceptor atoms in the molecules. The Hamiltonian acts
on two diabatic states and has a simple chemically motivated form for its
matrix elements. The model gives insight into the "H-bond puzzle", describes
different classes of bonds, and empirical correlations between the
donor-acceptor distance R and binding energies, bond lengths, and the softening
of vibrational frequencies. A key prediction is the UV photo-dissociation of
H-bonded complexes via an excited electronic state with an exalted vibrational
frequency.
