Computing Excited OH Stretch States of Water Dimer in 12-D Using Contracted Intermolecular and Intramolecular Basis Functions
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Due in part to the ubiquity and importance of water, water dimer has been intensively studied since it became possible to record its spectrum. Computing the (ro-)vibrational spectrum of water dimer is challenging. The water dimer potential has 8 equivalent wells separated by low barriers which makes harmonic approximations, and even numerical methods that begin with but improve a harmonic problem, of limited utility. A variational approach is imperative, but difficult because there are 12 coupled vibrational coordinate. In this paper, we use a product contracted basis, whose functions are products of intramolecular and intermolecular functions computed using an iterative eigensolver. An intermediate matrix F facilitates calculating matrix elements of the part of the potential that couples intra- and inter-molecular coordinates. Using F, it is possible to do calculations on a general potential with a huge quadrature grid without storing the potential on the full grid. We find that surprisingly many intermolecular functions are required. This is due to the importance of coupling (related to the hydrogen bond) between inter- and intra-molecular coordinates. The full G16 symmetry of water dimer is exploited. We calculate, for the first time, monomer excited stretch states and compare P(1) transition frequencies with their experimental counterparts. We also compare vibrational shifts and tunnelling splittings with those found by experimentalists. Surprisingly, we find that the the largest tunnelling splitting, which does not involve interchange of the two monomers, is smaller in the asymmetric stretch excited state than in the ground state. Differences between levels we compute and those obtained with a [6+6] adiabatic approximation [Leforestier et al. J. Chem. Phys. 137 014305 (2012) ] are ~ 0:6 cm -1 for states without monomer excitation, ~ 4 cm -1 for monomer excited bend states, and as large as ~ 10 cm -1 for monomer excited stretch states.