This is the real Deep Purple "Smoke on the Water" Riff
It is authentic. It will take a little while to get it right though.
You can play it either the 6th and 5th string, or the 5 and 4th 
whatever you want (6th and 5th are more authentic sounding)

(electric guitar with heavy distortion)
|--3----6----9-----3----6----10----9----3----6----9----6----3---------
|--5----8---11----5----8----12----11---5----8---11---8----5-----------
|-------------------------------------------------------------------------
|-------------------------------------------------------------------------
|-------------------------------------------------------------------------
|-------------------------------------------------------------------------

Another way of playing it, is this:
|--3-------------3--------------------3-------------------3------------
|--5----6----9---5----6----10----9----5----6----9---6-----5-----------
|-------8---11--------8----12----11--------8---11---8-----------------
|-------------------------------------------------------------------------
|-------------------------------------------------------------------------
|-------------------------------------------------------------------------

You can play this riff just about anywhere on the
guitar, and it'll sound fine. Just don't forget the distortion.

This only took me about a half of an hour, so if there
are any mistakes, oops.