- The pseudopotential method is used to calculate first- and second-order interaction parameters in f.c.c. and h.c.p. interstitial solutions of aluminium and magnesium alloys. Values calculated of interaction parameters are used for an estimation of the series convergence for the logarithm of the boron and carbon activity coefficient. The results show that the series convergence depends, to a great extent, on the selection of the alloying element and its concentration. [Russian Text Ignored].