  We demonstrate that the finite difference grid method (FDM) can be simply
modified to satisfy the variational principle and enable calculations of both
real and complex poles of the scattering matrix. These complex poles are known
as resonances and provide the energies and inverse lifetimes of the system
under study (e.g., molecules) in metastable states. This approach allows
incorporating finite grid methods in the study of resonance phenomena in
chemistry. Possible applications include the calculation of electronic
autoionization resonances which occur when ionization takes place as the bond
lengths of the molecule are varied. Alternatively, the method can be applied to
calculate nuclear predissociation resonances which are associated with
activated complexes with finite lifetimes.
