Carbynes phonons: a tight binding force field.

Modeling the vibrational structure of linear carbon chains has proved to be a difficult task with present first-principles calculations. This limits their applicability for the interpretation of experimental data, such as Raman scattering experiments on linear carbon chains within nanotubes. These limitations can be overcome by means of a simple tight binding scheme for pi-electrons. In this work a force field for the calculation of longitudinal phonon dispersion branches is built on the basis of bond-bond polarizabilities and just three parameters. The so obtained phonon dispersion branches are in very good agreement with the experimental data on carbynes in different environments and polyynes of any length. The model is discussed in relation to the importance of long range vibrational interactions in carbynes. The physical phenomena affecting their vibrational properties (i.e., Kohn anomaly, electron-phonon coupling) can be accurately and analytically described by the present approach.