Difference between 決{き}める and 決{けっ}する?


I'm reading a text that has the word 決{けっ}する, and so far I've only seen 決{き}める
to mean "decide". I've searched a few dictionaries and couldn't find the
difference between the two. Is there a difference or nuance I'm missing?




**決める**

  * a basic word that everyone knows
  * basically a weak and casual word like "make a choice", but can also represent stronger decisions (like "make up one's mind"); for authoritative decisions we usually choose other words

> ピカチュウ！ 君に決めた！  
>  _I choose you! Pikachu!_

  * **transitive** ; the intransitive counterpart is 決まる "be decided"

**決する**

  * a big, bookish word that wouldn't be uttered by a child
  * means to make a conclusive settlement (usually, of _yes-and-no_ or _either-or_ ) through the decision
  * transitive **or** intransitive 

> 運命を決する _(it) determines the fate_  
>  運命が決する _the fate will be determined_




決する mainly appears in some set phrases such as 勝負を決する,
[雌雄を決する](http://jisho.org/word/%E9%9B%8C%E9%9B%84%E3%82%92%E6%B1%BA%E3%81%99%E3%82%8B),
[意を決する](http://jisho.org/word/%E6%84%8F%E3%82%92%E6%B1%BA%E3%81%99%E3%82%8B).
決する can be used in legal or judicatory contexts, too, although it sounds more
stiff than 決める. [けっして](http://jisho.org/word/%E6%B1%BA%E3%81%97%E3%81%A6) with
a negative expression means "never", but it's probably better to regard this
as a distinct adverb.

Except for these, using 決する instead of 決める would sound fairly literary or even
funny. Don't say something like 明日どこに遊びに行くか決する, for instance.

