Ammonia quantum tunneling in cold rare-gas He and Ar clusters and factorial design approach for methodology evaluation

Quantum tunneling of the ammonia inversion motion and energy level splittings in He and Ar clusters were investigated. It was found that the double well potential (DWP) in He clusters is symmetrical and that the first layer of He atoms is able to model the system. The calculated tunneling splitting was in good agreement with the experimental, 36.4 and 24.6 cm(-1) respectively. For NH3 in Ar clusters, the DWP becomes slightly asymmetric, which is enough to decrease the resonance and make the symmetric DWP unable to model the system. An asymmetric potential was used and the result was in excellent agreement with the experimental splitting, of 9.0 and 10.6 cm(-1) respectively. Non-covalent interactions revealed that the asymmetry is caused by dissimilar interactions in each minimum of the double well potential. The effects of different methodologies were analyzed via a design of experiments approach. For the gas-phase NH3 molecule, only diffuse functions were statistically significant while for the NH3 embedded in He cluster both the MP2 method and polarization functions were significant. This tendency suggests higher order polarization functions may be essential to generate accurate barrier heights.

Automated summary

This study investigates the quantum tunneling of ammonia inversion motion and energy level splittings in helium and argon clusters. The results show that the double well potential in helium clusters is symmetrical, while in argon clusters it becomes slightly asymmetric. The calculated results are in good agreement with the experimental data, suggesting the effectiveness of the models used.