verify this result half quantitatively using a model kit as analog computer. The different sizes of C and Si are simulated with tetrahedral joints whose arm lengths differ[*] and the atoms are joined by flexible bonds (bent bonds). In disilabicyclo[l .1 .O]butane C,Si,H, (2) the region between the two C atoms does have a high ELF value (Fig. 1 c and 1 d). This confirms the previously described bond.['] The relatively small region of high ELF values implies a weak bond, in agreement with the long bond length. The white ELF maximum is also clearly off the straight topological CC connecting line. Its position is remarkably close to that of the bent bond derived from the simple structural model.IZ1 As expected, there is no bond between the Si atoms (Fig. 1 d). on the Cray-2 in Stuttgart. Mr. M. Kohout (Universitat Stuttgart) contributed to the development of the program MEROP (for the calculation of the electron density and of ELF) and wrote the program MPLOT (for drawing the contour lines of Fig. 2). The methods for obtaining localized orbitals-often used in the chemistry of molecules to describe bonding-can be used in principle for solids as well (in methane and in diamond , for example). They can lead, however, to several equivalent sets of orbitals for a given structure and are non-unique in this case. This ambiguity occurs, for example, in monomeric monocycles such as benzene, or in an infinite polyene chain."] In solids ambiguity often arises on account of the higher coordination, and localized orbitals are therefore used only rarely. An analysis in positional space can nevertheless be performed when instead of the equivocal localized orbitals, the electron localization function (ELF) is used. In this work we have calculated ELF for crystalline solids for the first time. The electron localization function was introduced by Becke and Edgecombe as a measure of the probability of finding an electron in the neighborhood of another electron with the same spin."] ELF is thus a measure of the Pauli repulsion. The explicit formulation is given in Equation (a) The parameter K is the curvature of the electron pair density for electrons of identical spin, e(r) the density at (Y), and Kh the value of K in a homogeneous electron gas with density e. The ELF values lie by definition between zero and one. Values are close to 1 when in the vicinity of one …