Which thermodynamic ensemble does Gaussian's BOMD functionality sample?
@pentavalentcarbon: As of now (Gaussian16), I believe that BOMD calculations can only be run in the NVE ensemble.

However, you can do ADMP (also available with Gaussian and uses an extended Lagrangian scheme), and offers equivalent functionality to BOMD. Thermostatting is available here, but the caveat is it implements only the simple velocity rescaling algorithm. On performing simple tests this seems to be "nearly" canonical on some systems I have worked on. i.e small ion-water clusters.