Study of infinite polyacetylene from a Heisenberg Hamiltonian: dimerization and lowest excitation energies

The infinite periodic polyacetylene is studied using a distance-dependent Heisenberg Hamiltonian extracted from ethylene and a recently proposed dressed cluster method. The ground state bond alternation is satisfactorily reproduced. The lowest excitation energies (of triplet and singlet character) are studied as a function of the bond alternation using excited roots of the dressed configuration interaction matrix and extrapolation techniques.