How is Aliphatic Index computed?

The aliphatic index of a protein is defined as the relative volume occupied by aliphatic side chains (alanine, valine, isoleucine, and leucine).
It may be regarded as a positive factor for the increase of thermostability of globular proteins.

Aliphatic index = ( (Ala count) + a * (Val count) + b * (Ile count) + b * (Leu count) ) * 100 where a = 2.9 and b = 3.9
Our approach is based on here.