Force corrections for the gas only. We compare the SGFC (exact value, in blue solid line) and the SLFC model for different smoothing lengths based on constant values for the SL (red, green and orange dashed lines) or on function є_g(d_g) (black cross markers). For a CSL, є_g = const., the SLFC and SGFC do not match at short separations: the error curve either tends either towards 0 or towards infinity. On the contrary, for a SVSL, є_g = є_g(d_g), the SLFC matches SGFC with an accuracy better than 0.5%. We found that (Appendix C). Top: SGFC and SLFC. Bottom: relative error between SGFC and SLFC (in %): 100 · |SLFC/SGFC − 1|.