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By the term “closed cluster” we mean a group of N atoms which reflects the structure of the crystal and furthermore the atom bonds which are present within the group - at the same time the torn bonds are absent. A similar procedure of closing the bonds known as “periodical boundary conditions’ is often used in one-dimensional atom chains when atom N+1 is identical to the first one. Such a procedure is also possible for 3D-systems, but is rarely applied here, since closing a 3D cluster is much more difficult. In 2D cases however, closing can be realized very easily - therefore, application of the CC approach to graphene and other 2D structures seems to be very effective.

While constructing clusters corresponding to graphene one must start out from it‘s crystal structure. The latter is well-known, a hexagonal layer which is formed by two sublattices, here A and B [@Wallace]. The simplest unclosed cluster corresponding to such a structure is shown in Fig.\[Fig1a\].

The main idea of the closed cluster approach is the necessity of closing the torn bonds shown in Fig.\[Fig1a\] in the way shown in Fig.\[Fig1b\]. In case of an endless graphene layer the structure of 