<p>The novel coronavirus (SARS-CoV-2) has
infected over 850,000 people and caused more than 42000 deaths worldwide as of
April 1<sup>st</sup>, 2020. As the disease is spreading rapidly all over the
world, it is urgent to find effective drugs to treat the virus. The main
protease (Mpro) of SARS-CoV-2 is one of the potential drug targets. In this
work, we used rigorous computational methods, including molecular docking, fast
pulling of ligand (FPL), and free energy perturbation (FEP), to investigate
potential inhibitors of SARS-CoV-2 Mpro. We first tested our approach with
three reported inhibitors of SARS-CoV-2 Mpro; and our computational results are
in good agreement with the respective experimental data. Subsequently, we
applied our approach on a databases of ~4600 natural compounds found in
Vietnamese plants, as well as 8 available HIV-1 protease (PR) inhibitors and an
aza-peptide epoxide. Molecular docking resulted in a short list of 35 natural
compounds, which was subsequently refined using the FPL scheme. FPL simulations
resulted in five potential inhibitors, including 3 natural compounds and two
available HIV-1 PR inhibitors. Finally, FEP, the most accurate and precise
method, was used to determine the absolute binding free energy of these five
compounds. FEP results indicate that two natural compounds, <i>cannabisin </i>A and <i>isoacteoside</i>, and an HIV-1 PR inhibitor, <i>darunavir</i>, exhibit large binding free energy to SARS-CoV-2 Mpro,
which is larger than that of <b>13b</b>, the most reliable SARS-CoV-2 Mpro
inhibitor recently reported. The binding free energy largely arises from van
der Waals (vdW) interaction. We also found that Glu166 form H-bonds to all the
inhibitors. Replacing Glu166 by an alanine residue leads to ~ 2.0 kcal/mol
decreases in the affinity of <i>darunavir </i>to
SARS-CoV-2 Mpro. Our results could contribute to the development of potentials
drugs inhibiting SARS-CoV-2. </p>