  Based on the geometrical approach to tunnelling by P.D. Hislop and I.M. Sigal
[Memoir. AMS 78, No. 399 (1989)], we introduce the concept of a leading
tunnelling trajectory. It is then proven that leading tunnelling trajectories
for single-active-electron models of molecular tunnelling ionization (i.e.,
theories where a molecular potential is modelled by a single-electron
multi-centre potential) are linear in the case of short range interactions and
"almost" linear in the case of long range interactions. The results are
presented on both the formal and physically intuitive levels. Physical
implications of the obtained results are discussed.
