Cannot versus can not

This isn't a phrase, but I was wondering where I could find the correct useage for cannot versus can not. Isn't there a 'hard fast rule' somewhere that sometimes you can not use the word cannot?
thanks

The hard-and-fast rule of usage is that "cannot" is always written as one word, with no space. If you're asking whether any situations call for using a space, I guess one sort of conversational context qualifies, like this: "Should I post my question at the Phrase Finder Discussion Forum? Well, I suppose I CAN post it and wait for an answer--or I can NOT post it and ask my neighbor the copy editor instead."

At a stretch, I suppose I might write "cannot" as two separate words only if writing dialogue, where there is heavy emphasis on the unacceptability of whatever the couse of action is. I'd probably embolden or capitalise or underline the "not" though. Think tennis, and McEnroe here... he yelled "You can *NOT* be serious, man!", with the stress very heavily on the third syllable of the sentence.