Approximate calculation of the collective excited states of a cytosine stack

Using the sum rules of oscillator strengths of different individual transitions of different symmetry, a simple algebraic equation for the frequency of the collective excitation of the pi-system, of only the sigma electrons, and finally for the whole valence electron system of a cytosine stack was derived. The input data (besides the geometry of DNA B and of the number of different types of electrons in cytosine) for the individual valence and core excitations were estimated on the basis of ab initio Hartree-Fock band structures of a cytosine stack. These bands were corrected for correlation at the MP2 level and the results of an intermediate exciton calculation were taken into account. The obtained frequencies of the different collective excitations (plasmons) are in fairly good agreement with the results of energy loss experiments.