We present an investigation of the vibrational density of states (VDOS) of selenium calculated using two different methods. First we compute the VDOS from the velocity autocorrelation and study its evolution with the temperature. Secondly we compute it from the dynamical matrix. This latter method leads to imaginary frequencies for finite temperatures. In the literature these frequencies are called unstable and are related to the diffusion and/or the anharmonicity. We investigate the character of these frequencies, asking whether they are localized or extended, more acoustic or optic. We show that, below a temperature-dependent threshold frequency, they are similar to their real low-frequency counterparts. The temperature evolution of the number of these modes changes at the glass transition.