Vibrational second-order perturbation theory (VPT2) using local monomer normal modes

Local normal modes have been used recently in vibrational self-consistent field/virtual state configuration interaction calculations of vibrational energies and infrared spectra of water clusters ranging from the water hexamer to bulk water. This approach is termed the local monomer model, and here we show that vibrational second-order perturbation theory (VPT2) can be used with this model. The approach is thus complementary to standard VPT2 theory, but has near linear scaling with the number of monomers, in contrast to standard VPT2 theory. The local VPT2 approach is described and then illustrated and tested for the water dimer H2O2 using the ab initio Wang-Huang-Braams-Bowman (WHBB) potential energy surface, mainly for the six intramolecular modes.