It is shown that the use of the simple cubature formula for the approximate calculation of the density matrix em in the non-local Hartree-Bock exchange for crystals leads to a non-physical divergence of the lattice sums. To overcome this difficulty it is proposed to introduce a weight function in the cubature formula for the numerical integration over the Brillouin zone in calculating non-local exchange. It is discussed the simple way t o choose the nodes of the numerical integration which is based on the large unit cell transformation in the direct and reciprocal lattices. This choice allows to extend the special point technique of the Brillouin zone integration to the non-local exchange calculation. pacxonHMocTn B pelueTosHbIx cyMMax 06MeHHOfi XIacTM n o~e~u~a n a XapTpH-cDoHa.

A new quantum chemical definition is proposed of the full atomic valence, V A , taking into account both the covalent and ionic parts of the chemical bonds formed by atoms in molecules and crystals. The full atomic valencies, covalencies, and charges on atoms are calculated for nickel-oxygen crystalline compounds in the CNDO band theory approximation. A comparison of the chemical bonding in nickel and copper crystalline oxides is given.

A Comparison with CuO and La,CuO, BY J. CHOISNET (a), R. A. EVARESTOV (b), 1. I. TUPITSIN (b), and V. A. VERYAZOV (b) Modelling from quantum chemical data of the chemical bonding in NiO and La,NiO, crystals comparatively to CuO and La,CuO,, is proposed. The increase of the ionicity degree of the Ni(I1)-O bonding in comparison with Cu(ll)-O bonding is observed. The covalency is low and equally distributed in the cquatorial plane and apical direction in the NiO, octahedra in La,NiO,. As a supplementary data, which is common to La,CuO, and La,NiO,, the covalency of the apical La(1II)-O bonding plays an important role in increasing the overall covalent character of the apical oxygen and triggering a further elongation of MO, octahedra.

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