In similar fashion, the energy of self-associating O-H…O HBs by MTA in (H 2 O) m (H 2 S) n (n + m = 2 to 8) clusters were calculated (cf. Tables ). Except for HB3 in (H 2 O) 6 (H 2 S) 1 -A), these O-H…O HB energies are found to be in the range 3.62 to 11.06 kcal mol -1 . These calculated O-H…O HB energies in (H 2 O) m (H 2 S) n (n + m = 2 to 8) clusters are seen to be similar or of little smaller in strengths as compared to those in pure water clusters, reported by Ahirwar et al. The respective O-H…O HB energy in the dimers are in the range of 3.99 to 5.69 kcal mol -1 . The enhancement in the O-H…O HBs strength due to the cooperative networking effect of other HBs is in between -0.62 and 5.96 kcal mol -1 ; except HB3 in (H 2 O) 6 (H 2 S) 1 -A cluster. The cooperativity contribution towards HB3 in (H 2 O) 6 (H 2 S) 1 -A is -1.26 kcal mol -1 . This may be due to the weaker nature of the hydrogen bond HB3 (large O-H…O distance (2.35 Å) Also, the energy of HB3 is smallest (0.10 kcal mol -1 ) possibly due to the structural arrangement where the acceptor water molecule simultaneously donates and accepts one hydrogen bond, while the donor water molecule also donates and accepts one hydrogen bond. This structural feature is denoted as AD(W D )W A (AD) and was attributed to the formation of weak HBs in water clusters. Indeed, the strength of HB3 in (H 2 O) 6 (H 2 S) 1 -A is the lowest (0.10 kcal mol -1 ) among all the other O-H…O HBs, it has been suggested in the literature that strong HBs have a large contribution to cooperativity.