summate vs sum

summate is not a noun while sum is a noun.

Word	Noun	Adjective	Verb	Adverb
summate	No	No	Yes	No
sum	Yes	No	Yes	No

As verbs, summate and sum are synonyms defined as:

Other synonyms of summate include add, add together, add up, sum up, tally, tot, tot up, total, tote up.

summate (noun)	sum (noun)
	the whole amount
	the choicest or most essential or most vital part of some idea or experience
	a quantity obtained by the addition of a group of numbers
	the final aggregate
	a set containing all and only the members of two or more given sets
	a quantity of money

summate (verb)	sum (verb)
determine the sum of	determine the sum of
form or constitute a cumulative effect	be a summary of