Theoretical Studies of Unusual Molecular Vibrational Dynamics Open Access

Wang, Yimin (2010)

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With the recent progress by Braams, Bowman and co-workers, potential energy surface and dipole moment surface of polyatomic systems with up to 10 atoms can now be constructed pretty routinely by doing linear least-squares fit to tens of thousands of scattered ab initio energies. The key feature of these surfaces is that the invariance with respect to all permutations of like atoms. Such property is of truly importance to study the highly fluxional systems such as H5+ . Additionally, by incorporating the permutational invariance explicitly into the function representation, we not only obtain a compact fitting basis, but also reduce the size of the electronic energy data-set to a great extent.

Taking advantage of the above fitting technique, we successfully developed a highly accurate full-dimensional potential energy surface of a nine-atom molecule, malonadehyde, based on 11147 near basis-set-limit frozen-core CCSD(T) electronic energies. This potential energy surface has been used in a full-dimensional quantum study of hydrogen-atom transfer reaction of malonaldehyde using diffusion Monte Carlo simulation and a variational method, and obtained tunneling splitting in excellent agreement with the experiment. We recently developed a full-dimensional, flexible potential energy surface for arbitrary numbers of water monomers built from ab initio 2- and 3-body potentials. These potentials are each permutationally invariant fits to roughly 30000 electronic energies. Tests of these potentials are made against direct high-level ab initio results for the water dimer, trimer and hexamer.

The last part of this work devotes to several simple models for post-harmonic quantum vibration analysis on the full-dimensional ab initio potential energy surface. We report a local-mode mode model to calculate OH-stretch fundamentals of water clusters and and a new local-monomer model that describes both anharmonic stretches
and bends. Furthermore, we present tunneling calculations using a one-dimensional Hamiltonian in the imaginary-frequency, rectilinear normal mode of a saddle point, for the zero angular momentum state. Finally, a similar one-dimension Hamiltonian in normal coordinate has also been used to perform Frank-Condon analysis of the ionization thresholds of the two isomers of C3H.

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