[Markovnikov]

Instead of calculations which you do with regular MO theory, PMO uses qualitative reasoning between the interacting orbitals.

The link probably isn't a good one at all. But couldn't really find any good ones from .edu 

[Mitch]

So qualitative MO is simply called PMO?

[Markovnikov]

Well, I'm sure there's something more to it than a qualitative part...

But from my course, it's called PMO...

[BluRay]

Just to murky the waters still further, why do both cyclopropane and propene exist? One has the triangle structure (which I too read as being an equilateral triangle), and the other the single/double bond complex?

Probably because of the same reason you can have both diamond and graphite: in the right conditions, a chemical can assume more than one form, because it has more than one local minimum of gibbs energy.

Please note the difference between an allotrope and an isomer.

Yes, but the principle is the same.