  We present the computation of effective refractive coefficients for
inhomogeneous two-component grains with 3 kinds of inclusions with ${\rm
m_{incl}=3.0+4.0i, 2.0+1.0i, 2.5+0.0001i}$ and a matrix with ${\rm
m_m=1.33+0.01i}$ for 11 volume fractions of inclusions from 0% to 50% and
wavelengths ${\rm\lambda}$=0.5, 1.0, 2.0 and 5.0 ${\rm \mu m}$. The
coefficients of extinction for these grains have been computed using a discrete
dipole approximation (DDA). Computation of the extinction by the same method
for grains composed of a matrix material with randomly embedded inclusions has
been carried out for different volume fractions of inclusions. A comparison of
extinction coefficients obtained for both models of grain materials allows to
choose the best mixing rule for a mixture. In cases of inclusions with ${\rm
m_{incl}}$=2.0+1.0i and 2.5+0.0001i the best fit for the whole wavelengths
range and volume fractions of inclusions from 0 to 50% has been obtained for
Lichtenecker mixing rule. In case of ${\rm m_{incl}=3.0+4.0i}$ the fit for the
whole wavelength range and volume fractions of inclusions from 0 to 50% is not
very significant but the best has been obtained for Hanai rule. For volume
fractions of inclusion from 0 to 15% a very good fit has been obtained for the
whole wavelength range for Rayleigh and Maxwell-Garnett mixing rules.
