placidness vs calmness

placidness and calmness both are nouns.

Word	Noun	Adjective	Verb	Adverb
placidness	Yes	No	No	No
calmness	Yes	No	No	No

As nouns, calmness is a hypernym of placidness; that is, calmness is a word with a broader meaning than placidness:

placidness (noun)	calmness (noun)
a feeling of calmness; a quiet and undisturbed feeling	steadiness of mind under stress
	a feeling of calm; an absence of agitation or excitement
	an absence of strong winds or rain