where the three interaction site models such as TIP3P (Transferrable Intermolecular Potential 3P), SPC (Simple Point Charge), and SPC/E (Extended Simple Point Charge) have been used extensively. Although, all these water models represent similar interaction site, they show significant differences in the bulk properties. Very recently, Onufriev's group developed a new water model, OPC which differs significantly in the charges and the bond length. This new water model resulting into 3-charge, 4-point rigid water model, reproduces a comprehensive set of bulk properties significantly more accurate than commonly used water models (see ref). Figure . Structure and parameters (bond length, angle, charges) for different water models.