[quigonkenny]

Well, obviously it isn't as simple as "rub some atoms together" to do it, since the only ones we've seem deal with flame are Mustang and Kimblee. And arguably Kimblee wouldn't count, since he's probably just using the ambient materials to form something that automatically combusts in atmosphere, or more likely, creating dual substances (notice the symbols on his hands are different from each other) that when added together explode. Mustang is straight up directing flame.

Note that he's not creating it, only directing it. That's why he has the spark gloves. In fact we've yet to see an alchemist create heat or cold, if memory serves (first anime doesn't count). So lava's probably out.

And Ed doesn't appear to be able to do anything, but pretty much just stuff we'd already seen that he could do beforehand, with the only difference being no need for a circle. Even the transmuting with a Stone, which he adapted to using his own soul like a Stone, appears to have been something he (and all other Amestrian alchemists) had been doing unwittingly for generations.