Favipiravir is an antiviral medication currently being trialled as a COVID-19 treatment. To help accelerate these efforts, we have performed a research for tautomers formations of favipiravir as possible RNA polymerase enzyme inhibitors and mitigating the virus ability. This study provides important electronic and optical properties of tautomers determined by density functional theory (DFT) and time-dependent density function theory (TD-DFT) calculations in gas phase and in water. A series of favipiravir derivatives was designed, and study the effect of the HOMO-LUMO energy gap on the efficacy of inhibitors. It has been determined that H-atom positions change and substituting fluorine (F) by hydroxyl (OH) group of tautomers affects the energy gap and dipole moment values. Among all compounds, the results have shown that Fb4 form with OH is most potent inhibitory activity in both gas phase and water. These investigations indicated that these tautomers could be potentially developed into drugs, but further investigations are still required to examine the cytotoxicity and consequent side reactions.

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