Figure (b) has the N36 nitrile and O38 distributions from the highest scoring mode, ≥ 66. O38 is an acceptor in a strong (1.00) hydrogen bond with CYS 145 and a weak (.32, .22) acceptor to GLY 143 and SER 144. The binding is high with narrowness in the distributions, except for a small bi-modal component with O38. The neighboring pyrrolodine N36 is hydrogen bonded (1.0, donor) to PHE 140. Figure (d) has the O38 hydrogen bonded (.77) to CYS 145 and weakly (.37) to SER 144. The pyrrolidine rotating about and the rest of the molecule fixed does occupy the highly interacting modes with hydrogen bonding that helps a covalent reaction with CYS 145. The ligands in this highest non-covalent binding mode set either have hydrogen bonds with the O38 or N36, but not both simultaneously. N19 and O12 are typically hydrogen bonded throughout in this mode, adding to the overall total interaction for pre-covalent binding. The motive of this work is to explain the docking protocol. Nirmatrelvir is a reversible covalent binder to Mpro, with energy of 7.17 kcal. A detailed study of the Nirmatrelvir interaction with Mpro, using this docking method, is presented elsewhere, but to complete the brief analysis of non-covalent Nirmatrelvir binding, an analogous 20000 dockings were calculated with the sulfur in CYS 145 covalently bonded to Nirmatrelvir,