A systematic fitting procedure for accurate force field models to reproduce ab initio phonon spectra of nanostructures

A fitting procedure is presented to use a valence force field model to generate the phonon spectrum of large nanostructures. This approach uses a relatively large number of parameters (∼50) in order to generate the accurate ab initio phonon spectrum. Since the emphasis is in the accuracy rather than the transferability, it can only be used in similar bonding environments. Because of this, a reliable and automatic fitting procedure is essential. We discuss the detailed aspects of the fitting procedure, including the stages of fitting, the type of ab initio values used for the fitting, the weighting factors for different quantities, the number of ab initio data points needed, as well as the uniqueness of the parameters. We found that the parameters cannot be determined uniquely, indicating interdependence of the parameters. Nevertheless, the different parameters resulted from different fits all give accurate phonon spectrum compared to ab initio results. We have used the fitted valence force field model to study the phonon spectra of CdSe nanowires.