Structural properties and quantum effects in protonated helium clusters. I. The ab initio interaction potential

Ab initio calculations are carried out, at various levels of treatment of correlation forces, for the orientation and distance dependence of the interaction between a helium atom and the structure of the ionic core found earlier in protonated He cluster studies, i.e. the linear (HeHHe)(+) triatomic molecular ion. Several aspects of the potential energy surface are discussed and specific details of the stable configuration analysed. The final energy surface is spline-fitted for further use in the stochastic calculations for the ground vibrational states of the small protonated clusters presented in a following paper. (C) 2000 Elsevier Science B.V. All rights reserved.