The gas rarefaction, gas molecule-molecule interaction and gas-surface scattering influence on the gas flow through a slit into a vacuum is investigated by the direct simulation Monte Carlo (DSMC) method. To study the gas molecule-molecule interaction influence on the gas flow we used the hard sphere (HS), variable hard sphere (VI-IS) and variable soft sphere (VSS) models defined for the inverse-power-law (IPL) potential and also the generalized hard sphere (GHS) model defined for the 12-6 Lennard-Jones (LJ) potential. Maxwell (specular-diffuse scheme), Cercignani-Lampis (CL) and Epstein approaches were used to simulate the gas-surface scattering. The results of computations of the mass flow rate in a wide range of rarefactions and distributions of the density, temperature and mass velocity, and streamlines are presented. This study demonstrates that the gas molecule-molecule interaction significantly interferes with the gas flow through a slit, while the influence of the gas-surface scattering is negligibly small. Our results are in agreement with the corresponding theoretical asymptotes, experimental and numerical data.