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The response of, e.g., nanoparticles and molecules, when excited optically or vibrationally can be concisely described using the respective response functions. While the latter can be probed via electronic structure calculations, most commonly based on DFT and TDDFT, these methods are computationally expensive and scale poorly with system size. This is particularly limiting when considering ensembles of molecules and/or nanoparticles. Alternatively, the combined response of such systems can be described in terms of the response functions of the underlying units and low-order multipolar coupling. This approach, the elements of which are outlined in the theory section, is taken in the strongcoca package.
strongcoca can parse the output of several different computational packages, including gpaw and NWChem. It then generates different internal representations of the response functions, which allow one to combine the results from several different sources, e.g., a calculation for a molecule from NWChem with a calculation for a metallic nanoparticle from gpaw. The former could employ TDDFT via the Casida approach and a hybrid exchange-correlation functional, while the latter could be carried out using RTTDDFT using a functional more appropriate for a metallic system. The general workflow and the integration of different sources for response functions is described in the workflow section.