Vibrational Coupled Cluster Computations in Polyspherical Coordinates with the Exact Analytical Kinetic Energy Operator

We
present the first use of curvilinear vibrational coordinates,
specifically polyspherical coordinates, in combination with vibrational
coupled cluster theory. The polyspherical coordinates are used in
the context of both the adaptive density-guided approach to potential
energy surface construction and in the subsequent vibrational coupled
cluster calculations of anharmonic vibrational states. Results obtained
based on the polyspherical coordinate parametrization are compared
to results obtained with the use of rectilinear vibrational coordinates,
namely, normal coordinates and hybrid optimized and localized coordinates
for the formaldehyde molecule. This comparison is carried out with
the full vibrational configuration interaction model, using the respective
fully coupled potential energy surfaces and untruncated kinetic energy
operators. The polyspherical coordinates are shown to facilitate an
acceleration of convergence for truncated methods when compared to
the use of normal coordinates. We furthermore report on calculations
on the hydrogen peroxide molecule in the polyspherical coordinate
parametrization. The polyspherical vibrational coordinates are shown
to perform very well, even for truncated methods, especially when
considering the difficulty that rectilinear vibrational coordinates
can exhibit in treating complicated internal molecular motion.