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Fig. 10. Contours of the energy difference [E(1'B,)-E(1'A,)] in the C», restricted coordinated space near point A. The interna coordinates are the Cartesian coordinates of the end atom (se¢ Fig. 3 of ref. [1]). Energy increment = 10 mh. Solid lines, > 0; dashec lines, <0; bold contour = 0 = intersection seam in C),. The dot indicated as A is the point where the intersection seam given by Figs. £ and 6 penetrates this x-y plane. Asterisk: see Sect. 4.5  We call the confluence A of the two seams a node. Here the standard dimensionality rules [3] regarding intersec- tions require a modification which is readily understood in terms of the derivation of these rules (see, for example, our discussion in Ref. [3b]): In the present case we have, in the region of interest, two surfaces on which Hj, = 0 (one of them being the C),-conserving

Figure 10 Contours of the energy difference [E(1'B,)-E(1'A,)] in the C», restricted coordinated space near point A. The interna coordinates are the Cartesian coordinates of the end atom (se¢ Fig. 3 of ref. [1]). Energy increment = 10 mh. Solid lines, > 0; dashec lines, <0; bold contour = 0 = intersection seam in C),. The dot indicated as A is the point where the intersection seam given by Figs. £ and 6 penetrates this x-y plane. Asterisk: see Sect. 4.5 We call the confluence A of the two seams a node. Here the standard dimensionality rules [3] regarding intersec- tions require a modification which is readily understood in terms of the derivation of these rules (see, for example, our discussion in Ref. [3b]): In the present case we have, in the region of interest, two surfaces on which Hj, = 0 (one of them being the C),-conserving