Multilevel Approach for Direct VSCF/VCI MULTIMODE Calculations with Applications to Large "Zundel" Cations

We test existing efficient schemes for the "direct-dynamics" approach in building a potential energy surface (PES) in the code MULTIMODE. These are (1) the n-mode representation (nMR) approach to the PES, (2) the exploitation of the normal mode's symmetry to reduce the computational effort in constructing the PES, (3) the use of sparse grids for fitting the n-mode potentials, and (4) different levels of ab initio theory for these potentials. These schemes are applied to a four-dimensional calculation for the proton-bound methanol dimer (CH₃OH)₂H⁺. In addition to the major reduction in complexity obtained by considering only four modes, the combination of these schemes leads to a significant reduction in the computational effort without any major loss of accuracy. VSCF/VCI test calculations are presented for (CH₃OH)₂H⁺.