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Non-rigid molecules
A significant portion of the molecules dealt with in the team are non-rigid molecules displaying large amplitude motions. For these molecules a specific treatment is necessary as their vibrational energies cannot be obtained using the harmonic approximation nor
can their rotational energies be calculated using the rigid rotator approach.
To treat such molecules, 4-D models are used in which the large amplitude motion and the overall rotation are treated simultaneously. Such models were applied to the internal rotation of bideuterated methanol CH2DOH and for the quasi-linearity of HCCN, CCN, CH2, and H2O.
Effective 3-D models are also used in which the Coriolis coupling between the large amplitude motion and the overall rotation is accounted for. Such a treatment was used in the case of monodeuterated acetaldehyde CH2DCOH.
Triatomic molecules, displaying anomalous centrifugal distortion, were also modeled based on their exact Hamiltonian. In this 6-D approach, matrix elements are evaluated using Gaussian quadrature. A potential energy surface, usually obtained through ab initio calculations, is also required.
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