summation vs summational

summation is a noun but summational is not a noun.

Word	Noun	Adjective	Verb	Adverb
summation	Yes	No	No	No
summational	No	Yes	No	No

summation (noun)	summational (noun)
the arithmetic operation of summing; calculating the sum of two or more numbers
a concluding summary (as in presenting a case before a law court)
the final aggregate
(physiology) the process whereby multiple stimuli can produce a response (in a muscle or nerve or other part) that one stimulus alone does not produce

summation (adjective)	summational (adjective)
	of or relating to a summation or produced by summation