Avian influenza or 'Bird flu' is a contagious disease of animals caused by viruses. Present day drugs are able to reduce the time to symptom resolution by 1.5 days provided therapy is started within 48 hours of the onset of symptoms, and have certain limitations. There is a need to develop a new drug that has high efficacy than present day drugs, by computer aided drug designing, binding affinity calculations including molecular mechanics based free energy calculations. Energy components calculated by performing molecular mechanics calculations both in explicit solvent and complex states are sufficient to estimate the relative binding free energy differences between two inhibitors qualitatively.