In all three systems the A-site cations show the greatest contribution to the low-frequency acoustic modes. Heavier cations produce lower-frequency, sharper peaks in the PDoS consistent with "rattling" behaviour, indicating weaker interactions between the A-site cations and the TiO 6 octahedral framework. The higherfrequency optic modes are predominantly due to the motion of the Ti and O atoms and correspond to deformations of the TiO 6 octahedra. In all three systems the optic modes are separated into a broad band of mid-frequency modes involving motion of the Ti and O atoms and an isolated group of high-frequency modes associated primarily with O. The calculated phonon spectra of STO and CTO agree well with previous calculations. While we were not able to find reference phonon spectra for R3m BTO to compare to, the good agreement for STO and CTO gives us confidence in our calculation on BTO.