The applicability of the perturbation theory to studies on the effect of confinement is discussed in the context of the influence of spatial confinement on the Rydberg Heft molecule. In the analysis the confining potential is assumed to have cylindrical symmetry, with symmetry axis of the potential overlapping with the molecular axis. In the direction perpendicular to the axis the quadratic dependence of the potential on the electron coordinates is assumed. The ionization due to the confinement and the influence of the confining potential oil the form of the potential energy curves, in particular on the bond lengths, are studied in detail. (c) 2006 Elsevier B.V. All rights reserved.