  The resonant frequencies of a structure and the associated field
distributions are generally determined by solving a non-linear eigenvalue
problem. Using frequency-domain solvers, the response of the structure needs to
be evaluated at many different frequencies in order to solve the non-linear
problem. Moreover, these frequencies may be complex, the inverse of the
imaginary part physically corresponding to the e-folding time of the energy of
the mode. In this paper, we propose to use the so-called "Geometrical Method of
Moments" (GMoM) to accelerate the computation of the resonant frequencies of a
structure using the Method of Moments. First, purely geometrical reaction
integrals are precomputed, which do not depend on the frequency nor material
parameters. Then, by summing these terms with proper weights, the impedance
matrix can be obtained for any complex frequency. This method easily
accommodates for dispersive materials provided that the permittivity and
permeability of the material can be extrapolated to complex frequencies.
