A potential-energy force field for methanol is calculated at the coupled-cluster singles doubles and noniterative triples correction level using the correlation-consistent polarized valence triple zeta basis set. The force field describes the coupled molecular vibrations, including the cubic and quartic anharmonicites, as a function of the torsional coordinate. The resulting molecular eigenstates are calculated using a combination of perturbative and variation calculations following the approach of Castillo-Chará and Sibert [J. Chem. Phys. 119, 11671 (2003)]. The energies, including torsional tunneling splittings, are compared with available spectroscopic data for all the fundamentals. Excellent agreement is found. Varying the torsional mass, correlation plots are constructed in order to elucidate the complex role of torsion-vibration coupling.

A fully three-dimensional morphed potential energy surface is reported for Ar:HBr. The morphed potential was obtained from parametrized scaling and shifting transformations of an ab initio potential. The optimum parameters of the morphed potential were determined by a regularized nonlinear least-squares fit to available experimental data. The rovibrational dynamics of the complex were computed using an adiabatic separation of the H-Br intramolecular stretching mode from the intermolecular modes of the system. The ground rovibrational state of the morphed potential was found to have the hydrogen-bound structure Ar-HBr. This state was 10.99 cm Ϫ1 more stable than the corresponding state having the van der Waals structure, Ar-BrH, in agreement with experimental data. However, the global minimum of the morphed potential was found to have the van der Waals structure, Ar-BrH. This structure was 20.9 cm Ϫ1 lower in energy than the local minimum having the hydrogen-bound structure, Ar-HBr.

Effect of solvent on molecular conformation: Microwave spectra and structures of 2-aminoethanol van der Waals complexesAb initio relativistic all-electron calculation of the Ar-I 2 ground state potential A study of the ArCl 2 Van der Waals complex: Ab initio-based potential energy surfaces, the relative stability of conformers, and the "hidden" microwave spectrumThe structure and ground state dynamics of the atom-diatom dimer interaction between Ar and HI has been investigated by microwave and near infrared supersonic jet spectroscopy. Ab initio molecular orbital calculations were used to provide greater insight into the nature of the interaction. The ground state is shown to be in the isomeric form Ar-IH with R cm ϭ3.9975(1) Å, ϭ149.33(1)°for the normal isotopomer and R cm ϭ3.9483(1) Å, ϭ157.11(1)°for Ar-ID. The potential surface from an ab initio molecular orbital calculation was scaled and shifted to yield a nonlinear least-squares fit of the rovibrational state energies to the experimental data. The ground state potential energy surface obtained in this manner has a barrier between the Ar-IH and Ar-HI isomers of 88.5 cm Ϫ1 with respect to the global minimum. Such calculations are also used to predict the presence of localized states in the secondary minimum associated with isomers Ar-HI and Ar-DI. Attempts to experimentally identify transitions associated with the latter were unsuccessful. The ground state, Ar-IH isomeric structure, contrasts with the corresponding ground state of the other members of the homologous series Ar-HX ͑XϭF, Cl, and Br͒ in which the Ar is bound to the proton.

The vibrations of methanol are studied theoretically via a fully coupled treatment of the small amplitude motions and the large amplitude torsional mode. Van Vleck perturbation theory is used to find a representation in which the coupling between the vibrational states is reduced. The full vibration-torsion eigenstates are obtained subsequently via matrix diagonalization. The convergence of the perturbation theory and variational calculations are discussed. The torsion-vibration energies and the torsional tunneling splittings are compared with available spectroscopic data for all the fundamentals. The unusual inverted E-A torsional tunneling splitting pattern observed spectroscopically for rotationless torsion-vibrational states is predicted by our calculation.

Supersonic jet investigations of the (HBr)(2) dimer have been carried out using a tunable diode laser spectrometer to provide accurate data for comparison with results from a four-dimensional (4-D) ab initio potential energy surface (PES). The near-infrared nu(1) (+/-), nu(2) (+/-), and (nu(1)+nu(4))(-) bands of (H (79)Br)(2), (H (79)Br-H (81)Br), and (H (81)Br)(2) isotopomers have been recorded in the range 2500-2600 cm(-1) using a CW slit jet expansion with an upgraded near-infrared diode laser spectrometer. The 4-D PES has been calculated for (HBr)(2) using second-order Møller-Plesset perturbation theory with an augmented and polarized 6-311G basis set. The potential is characterized by a global minimum occurring at the H bond structure with the distance between the center of masses (CM) of the monomer being R(CM)=4.10 A with angles theta(A)=10 degrees, theta(B)=100 degrees and a well depth of 692.2 cm(-1), theta(A) is the angle the HBr bond of monomer A makes with the vector from the CM of A to the CM of B, and theta(B) is the corresponding angle monomer B makes with the same CM-CM vector. The barrier for the H interchange occurs at the closed C(2h) structure for which R(CM)=4.07 A, theta(A)=45 degrees, theta(B)=135 degrees, and the barrier height is 73.9 cm(-1). The PES was fitted using a linear-least squares method and the rovibrational energy levels of the complex were calculated by a split pseudospectral method. The spectroscopic data provide accurate molecular parameters for the dimer that are then compared with the results predicted on the basis of the 4-D ab initio PES.