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Presents a method for constructing computationally cheap adiabatically corrected potential energy surfaces (PES) for intermolecular vibrational states. Reproduction of published experimental data for previous vibrational states; Comparison made between a set of intermolecular PES with a chemical core frozen into the equilibrium geometry.

The rates of rotational transitions for HCO+, the most abundant ion in interstellar space, induced by collision with helium are obtained for temperatures ranging from 10 to 80 K. The calculations are based on a new potential energy surface for the He–HCO+ interaction and on a scattering matrix whose accuracy was checked by pressure broadening and shift measurements. The rates decrease for increasing values of j and , with a temperature trend depending on the energy involved in the transitions: if it is small, the rates are almost constant, while an increase with T is found for other cases. Comparison with previous and less accurate results shows an agreement within 50 per cent. Comparison between state-to-state and pressure broadening cross-sections allows us to discuss importance and influence of elastic and inelastic collisions. [ABSTRACT FROM AUTHOR]

Using a recoupling technique with close-coupling spin-free calculations de-excitation rate coefficients are obtained among hyperfine transitions for He colliding with N2H+. A recently determined potential energy surface suitable for scattering calculations is used to investigate rate coefficients for temperatures between 5 and 50 K, and for the seven lowest rotational levels of N2H+. Fitting functions are provided for the Maxwellian averaged opacity tensors and for the rotational de-excitation collisional rate coefficients. The fitting functions for the opacity tensors can be used to calculate hyperfine (de)-excitation rate coefficients among elastic and inelastic rotational levels, and among the corresponding magnetic sublevels of the hyperfine structure. Certain dynamical approximations are investigated and found to be invalid. [ABSTRACT FROM AUTHOR]

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH[sub 4] [sup +] -Ar, is calculated by ab initiomethods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ, X = D/T/Q). The deformation of the ammonium ion in the complex is shown to be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of the potential corresponds to a proton-bound structure with C[sub 3] symmetry (Rapproximately 3:4 A, Dapproximately 950cm[sup -1] ) and the barrier to internal rotation between the four equivalent minima is around 200cm[sup -1] . The three-dimensional potential is expanded in tetrahedral harmonics whose radially dependent coefficients, V(R), are compared for the considered levels of theory. The rotation-intermolecular vibration Hamiltonian is solved using a two-dimensional fixed-Rrepresentation of the calculated potentials, Vequals V(R), where the effective intermolecular separation, R[sup eff] , is determined from the experimental rotational constants of the complex. The accuracy of these parametrized potential energy surfaces is judged by their ability to reproduce the hindered rotor subband structure in the experimental upsilon[sub 3] (t[sub 2] ) infrared band of the complex. The simulations using the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of theory reproduce the coarse structure of the experimental spectrum well. Further improvement could be achieved by least-squares fitting the potential parameters to the observed subband positions. The fitted V[sub 3] and V[sub 4] parameters remain in close agreement with those determined from the ab initiocalculations but the anisotropy of the potential is significantly different from that in a previous least-squares fit of V[sub 3] alone. [ABSTRACT FROM AUTHOR]