Prominent among the classes of collective excitations in liquids that one would like to study are those which are compelled to obey some sort of conservation law. The instantaneous normal modes of liquid (which must be translationally invariant or, equivalently, conserve momentum) comprise one such example. The set of relaxation pathways dictated by a master‐equation description of energy transfer in a liquid—which must conserve probability—constitutes another. We show that these conservation laws do impose fairly stringent requirements on the nature of the collective behavior, but the resulting excitations can nonetheless be described by liquid‐theory methods. Within linear liquid theories, the desired distribution of modes ends up being a combination of a delocalized electronic‐band‐like portion and a fluctuating local field contribution. We illustrate the results with an explicit calculation (at the master‐equation level) of energy‐transfer kinetics in a liquid.
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