AN AB-INITIO APPROACH TO DETERMINING THE PARAMETERS OF THE MODEL HAMILTONIAN (H)OVER-CAP

By means of ab initio total-energy pseudopotential calculations, the p arameters of the model Hamiltonian for trans-(CH)(x) are determined. T he structural parameters of the ground state are obtained by relaxing the positions of the atoms and determining the lowest energy states fo r different lattice constants. The results are in agreement with exper iments. Then, applying constraints to the system, we calculate the coh esive energies of dimerization for various dimerized displacements. Co mparing these cohesive energies with those given by the model Hamilton ian, we determine the effective interaction strengths of the model Ham iltonian. With the above-mentioned parameters, the localized states of solitons are calculated. We find six localized soliton modes, three w ith odd parity and three with even parity. The former and the latter a re, respectively, qualitatively consistent with the Raman and infrared spectra of the material.