We have a new tutorial on Okapi Simple BM25 with Extension to Multiple Fields.
09-29-2016 Update: Sorry! Wait for an updated version of tutorial
Unlike the BM25, this model (known as Simple BM25F) incorporates the structure of documents into the scoring process.
In addition, we’ve uploaded a new, improved, and expanded version of the Okapi Best Match 25 tutorial.
Have a great IR day!