Using the semidirect variational method, the time dependent coefficients of a modal expansion of the neutron fluxes are given by the Euler-Lagrange equations obtained from a variational principle for the multigroup kinetic equations. In order to avoid the difficult task of determining orthogonal eigenfunctions for a modal expansion in a complex geometry, an approximate solution of the kinetic equations by the method of the “Green's Function” results in a set of readily calculated space modes. These modes can also be adapted to perturbations in the diffusion parameters about which one has some a priori information.