Reaction path Hamiltonian: Tunneling effects in the unimolecular isomerization HNC→HCN

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Abstract
The reaction path Hamiltonian model recently formulated by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)] is applied to the unimolecular isomerization HNC→HCN. The reaction path, and the energy and force constant matrix along it, are calculated at the SCF level with a large basis set (’’double zeta plus polarization’’), and the microcanonical rate constant k(E) is computed in the energy range near and below the classical threshold of the reaction. It is seen, for example, that rates as fast as 105 sec−1 can occur at energies ∼8 kcal/mole below the classical threshold.