Synonyms for Calm Down (same or very similar meaning)
WordNet sense 1 (make calm or still):
calm, lull, quiet, still, tranquilize, tranquillize, quieten, tranquilliseWordNet sense 2 (become quiet or calm, especially after a state of agitation):
cool off, chill out, cool it, simmer down, settle downFrom the ODE community, based on WordNetadd/edit