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We report the supermolecular interactions, electronic absorption spectra, and dynamic second-order nonlinear optical polarizabilities β of π-stacking (p-) and hydrogen-bonded (h-) dimers and a trimer in a 2-methyl-4-nitroaniline (MNA) crystal, using the TDB3LYP/3-21G+ coupled with the SOS methods. The calculated results show that the supermolecular interactions and second-order optical polarizabilities are smaller for dimers (p- and h-dimers) than the trimer in MNA crystal, which originates from size effect, and the h-dimer has smaller supermolecular interaction and larger second-order nonlinear optical response compared with that of the p-dimer, which originates from site effect in MNA crystal. The lowest absorption peak comes from the π−π* charge transfers within a MNA molecule for h-dimer, and between the two MNA molecules for the p-dimer in a MNA crystal. The calculated polarizability norm ||β|| of trimer consisting of h- and p-arrangements is 42.0 × 10-30 cm5 esu-1, and the macroscopy susceptibility norm ||χ|| is 6.15 × 10-7 esu as compared with the experimental value ||χ|| of 9.06 × 10-7esu in MNA crystal. The slipped cofacial arrangements of dimer (h-dimer) have larger contributions to the electronic absorption spectrum and nonlinear optical response of MNA crystal.