We obtain an eigenmode expansion of the electromagnetic Green’s tensor G(r,r') for lossy resonators in open
systems, which is simple yet complete. This enables rapid simulations by providing the spatial variation of G0(r,r') over both r and r' in one simulation. Few eigenmodes are often necessary for nanostructures, facilitating
both analytic calculations and unified insight into computationally intensive phenomena such as Purcell
enhancement, radiative heat transfer, van der Waals forces, and Förster resonance energy transfer. We bypass
all implementation and completeness issues associated with the alternative quasinormal eigenmode methods, by
defining modes with permittivity rather than frequency as the eigenvalue. Thus, modes decay rather than diverge
at infinity, and are defined by a linear eigenvalue problem, readily implemented using any numerical method.
We demonstrate its general implementation in COMSOL Multiphysics, using the default in-built tools.