By means of ab initio total-energy pseudopotential calculations, the parameters of the model Hamiltonian for are determined. The structural parameters of the ground state are obtained by relaxing the positions of the atoms and determining the lowest energy states for different lattice constants. The results are in agreement with experiments. Then, applying constraints to the system, we calculate the cohesive energies of dimerization for various dimerized displacements. Comparing these cohesive energies with those given by the model Hamiltonian, we determine the effective interaction strengths of the model Hamiltonian. With the above-mentioned parameters, the localized states of solitons are calculated. We find six localized soliton modes, three with odd parity and three with even parity. The former and the latter are, respectively, qualitatively consistent with the Raman and infrared spectra of the material.
Relation:
Journal of physics Condensed matter: an Institute of Physics journal 8(15), p.2539
