  We present and discuss a new computationally inexpensive method to study,
within the single active electron approximation, the interaction of a complex
system with an intense ultrashort laser pulse. As a first application, we
consider the one photon single ionisation of the highest occupied molecular
orbital of the water molecule by a laser pulse. The ionisation yield is
calculated for different orientations of the molecule with respect to the field
polarization axis and for different carrier envelope phases of the pulse, and
compared against predictions of another single active electron approach.
