Filter diagonalization (Wall, M. R.; Neuhauser, D. J. Chem. Phys. 1995, 102, 8011) is a new and efficient method for extracting frequencies and damping constants from a short-time segment of arty time-dependent signal, whether of quantum nature or not. In this letter, we will demonstrate that filter diagonalization can be used to follow the anharmonic vibrational dynamics of Si-6. Using the technique, we can locate (resolve) the normal modes (including the widths) from even a very short-time correlation signal generated by ab initio molecular dynamics calculations. Our results show that filter diagonalization can reduce significantly the necessary sampling time with an ab initio molecular simulation and aid in understanding normal-mode dynamics for systems that are affected by anharmonicity.