  A general variational approach for computing the rovibrational dynamics of
polyatomic molecules in the presence of external electric fields is presented.
Highly accurate, full-dimensional variational calculations provide a basis of
field-free rovibrational states for evaluating the rovibrational matrix
elements of high-rank Cartesian tensor operators, and for solving the
time-dependent Schr\"odinger equation. The effect of the external electric
field is treated as a multipole moment expansion truncated at the second
hyperpolarizability interaction term. Our fully numerical and computationally
efficient method has been implemented in a new program, RichMol, which can
simulate the effects of multiple external fields of arbitrary strength,
polarization, pulse shape and duration. Illustrative calculations of two-color
orientation and rotational excitation with an optical centrifuge of NH$_3$ are
discussed.
