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Helium clusters doped with diatomic molecules, HeN–BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as “electrons” and dopants as “nuclei” in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born–Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the 4He2–Br2(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N2(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced. [ABSTRACT FROM AUTHOR]

The interaction of He with Br[sub 2] in electronically excited B [sup 3]Π[sub u] state is investigated using spin-unrestricted single and double coupled-cluster approach with noniterative perturbative treatment of triple excitations. Internal electrons of the Br atom are described by effective core pseudopotentials. The validity of this approach is analyzed by comparing the lowest [sup 2]Σ[sup +] and [sup 2]Π electronic states of the HeBr molecule with those obtained in all electron calculations [J. Chem. Phys. 115, 10438 (2001)]. In this context, we examine the performance of different basis sets and saturation with bond functions. The comparison of theoretical blue-shifts with the experiment provides confidence about the present ab initio calculations. In addition, He–Br results of ab initio calculations at the same level are used to obtain approximate He–Br[sub 2] ([sup 3]Π[sub u]) interactions in the framework of the diatomics-in-molecule first order perturbation theory (IDIM-PT1) [J. Chem. Phys. 104, 9913 (1996)]. Overall, the IDIM-PT1 model results show a good agreement with the ab initio ones, being the main difference the sensitivity to the elongation of the Br–Br bond. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate, highly correlated calculations have been carried out for the ground electronic state and for a few of the lower excited electronic states, two of which are discussed in this work, of the trimer ionic helium cluster. Both linear asymmetric and nonlinear, symmetric, and asymmetric, configurations have been considered over a rather broad range of nuclear geometries. The results confirm the experimentally found [Chem. Phys. 102, 2773 (1995)] fragmentation patterns involving He and He+ fragments only and further suggest a range of possible nuclear geometries from which nonadiabatic couplings could also lead to He2+ fragments, albeit with a lower probability than the former channels. [ABSTRACT FROM AUTHOR]

A quantum chemistry-like approach has been recently developed in our group to deal with HeN–BC doped helium clusters, where the BC dopant is a conventional diatomic molecule. The central idea is to consider the He atoms as “electrons” while the B and C atoms play the role of the nuclei in standard electronic structure calculations. The procedure provides energies and wavefunctions allowing to perform spectral simulations and, hence, making feasible to do proper comparisons with current experiments. However, because of the large difference of masses of He and electrons, and also to the replacement of Coulomb potentials by molecular interactions, it is worthy to assess to what extent the approximations involved in this model (decoupling of orbital angular momenta of the He atoms from the BC rotation and adiabatic separation of the BC stretch versus the He motions) lead to accurate results. In this work we address these issues on the 4He2–Br2(X) system, containing a couple of bosonic He atoms for which variational calculations can be performed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]