**4. Conclusions**

Density functional theory calculation using DFT/B3LYP/6-31 + G (d) level of theory has been performed for four histamine H2 receptor antagonists, cimetidine, famotidine, nizatidine, and ranitidine with the help of Gaussian software. Vibrational analysis (IR) of four histamine H2 receptor antagonists has been generated by using DFT. Generated vibrational results of nizatidine were compared with the experimental result and the computed vibrational results found to be in good agreement with the experimental result. From thermochemical parameters of histamine H2 receptor antagonist, it was found that famotidine is having superior thermodynamics parameters among the four with high free energy, zero-point energy, enthalpy and low entropy. Comparing the global descriptive parameters of histamine H2 receptor antagonist, nizatidine is found to be having higher softness, ionization potential, electron affinity, chemical potential and lower hardness, which shows that it is more stable and chemically reactive. Higher value of electrophilicity index indicates its high biological activity. Nizatidine is found to be having lower energy gap, which shows that nizatidine is more chemically reactive. In case of stability, reactivity and biological activity ranitidine comes next to nizatidine and among them, cimetidine is less stable and having lower biological activity. At the same time, the Gibb's free energy revealed that the solubility of nizatidine is not sufficient to have adequate bioavailability. Finally, we could amorphousize nizatidine by quench cooling technique and found that it stable in amorphous state without showing any recrystallization tendency during super cooling and the subsequent heating in the metastable state. Nizatidine has a glass transition temperature around 282.1 K and was found to be stable over the measured temperature range up to 307 K.
