calm vs calm down

calm is a noun but calm down is not a noun.

Word	Noun	Adjective	Verb	Adverb
calm	Yes	Yes	Yes	No
calm down	No	No	Yes	No

As verbs, calm and calm down are synonyms defined as:

Other synonyms of calm include lull, quiet, quieten, still, tranquilize, tranquillise, tranquillize.

As verbs, calm and calm down are synonyms defined as:

calm and calm down: become quiet or calm, especially after a state of agitation

Other synonyms of calm include chill out, cool it, cool off, settle down, simmer down.

calm (noun)	calm down (noun)
steadiness of mind under stress
wind moving at less than 1 knot; 0 on the Beaufort scale

calm (adjective)	calm down (adjective)
(of weather) free from storm or wind
not agitated; without losing self-possession

calm (verb)	calm down (verb)
cause to be calm or quiet as by administering a sedative to	become quiet or less intensive
make steady	make calm or still
make calm or still	become quiet or calm, especially after a state of agitation
become quiet or calm, especially after a state of agitation