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A method based on the system transfer operator spectrum is proposed which reveals inherent properties of plasmonic nanostructures, without restrictions imposed by the form of incidence. T-matrix formulation is combined with eigenvalue theory for the first time, to the best of our knowledge. The electric field is directly calculated, in the whole space. Powers are formulated from eigenvalues and coupling coefficients, resulting in quantitative criteria for comparing different structures. Being semi-analytical makes it very fast and suitable for synthesis. A previously reported structure consisting of three Ag nanospheres is analyzed. Not only do the extracted plasmon modes validate our formulation, but the nature of the modes is also fully studied.