text: Davydov splitting is caused by excitations of these systems within individual unit cells.
full_question: Davydov splitting is caused by excitations of these systems within individual unit cells. The energy of two of these systems is proportional to 1 minus 3 times cosine squared of the angle between them, giving a critical value at 54.74 degrees. In linear optics, one of these things is created by an external field, with a magnitude proportional to the polarizability. The potential from this arrangement drops off with the (*) third power of distance. The potential energy of these objects is given by the dot product of the external electric field with their namesake moment, which is the product of charge and distance, and is symbolized mu. For 10 points, name these systems of a positive charge separated from a negative charge.
first_sentence: Davydov splitting is caused by excitations of these systems within individual unit cells.
raw_answer: electric dipoles [or electric dipole moment]