抄録
The exact expressions for the volume-dependent cluster sums (composed of four or less molecules) for the two-dimensional square lattice gas (in a square-shaped vessel) with nearest and second nearest neighbour interactions are obtained. The method used consists of the calculation of the exact partition functions and the calculation of the cluster sums from them with the use of the inverse relation. The detailed behaviour of the cluster sums as functions of the volume is investigated for the case when the nearest neighbour interaction is repulsive and the second nearest neighbour interaction is attractive. Finally a remark is given on the condensation of such a lattice gas.
