  The Stark resonance parameters for the $3a_{1}$ molecular orbital of H$_{2}$O
are computed by solving a system of partial differential equations in spherical
polar coordinates. The starting point of the calculation is the quantum
potential derived for this orbital from a single-center expanded Hartree-Fock
orbital. The resonance positions and widths are obtained after applying an
exterior complex scaling technique to describe the ionization regime for
external fields applied along the two distinct $\hat{z}$ directions associated
with the symmetry axis. The procedure thus avoids the computation of
multi-center integrals, yet takes into account the geometric shape of a
simplified molecular orbital in the field-free case.
