Benchmarks for the generation of interaction potentials for scattering calculations: applications to rotationally inelastic collisions of C-4 (X-3 Sigma(-)(g)) with He

We present an application of recently developed, explicitly correlated, partially spin-restricted coupled-cluster RCCSD(T)-F12x (x = A/B) methods [G. Knizia, T. B. Adler, and H.-J. Werner, J. Chem. Phys., 2009, 130, 054104] for the generation of multi-dimensional potential energy surfaces (PESs) for scattering calculations. We test the method on the O-2-He van der Waals model system by a comparison with standard orbital-based coupled-cluster techniques, employing correlation-consistent atomic basis sets (aug-cc-pVXZ, X = T, Q, 5, 6) and a complete basis set. From this comparison, it is obvious that the RCCSD(T)-F12/aug-cc-pVTZ approach is accurate enough for the description of short and long-range interactions with low computational effort. We apply this new method in studies of the interaction of the carbon-rich interstellar species C-4(X-3 Sigma(-)(g)) with atomic He. This PES is subsequently used in quantum close-coupling scattering calculations. The collisional excitation cross-sections of the fine-structure levels of C-4 by He are calculated at low collisional energies. The thermal dependence of rate coefficients is calculated up to 50 K. The propensity rules between fine-structure levels are studied, and it is shown that F-conserving cross sections are much larger, especially for high-N rotational levels rather than F-changing cross sections, as expected from theoretical considerations. This is the first report on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C4 and modeling of carbon-rich media.