finding $\varepsilon$: $$\label{eq:6}
\varepsilon^4 (\varepsilon^2 - 9) = 0.$$

The solutions of $\pm$3,0,0,0,0 are exactly those energy values at bottom, top and Dirac points of two energy bands, which are obtained from a tight binding and nearest neighbor approximations \[5\].

The fact, that the CC approach provides exact values of energy band boundaries already at a minimal cluster size is very important in the calculation of the energy of impurity states, as the energy of such states are counted from the bands’ boundaries.

Let us consider the next example with $N=12$. If we proceed from the clusters from Fig.\[Fig2\], then the secular equation has the form\

$$\label{eq:7}
 D_{12} =   \left|   
 \begin{array}  {rrrrrrrrrrrrr} 

     \varepsilon & 1 &   &   &   & 1 &   &   &   &  &  & 1\\
     1 & \varepsilon & 1 &  &  &  &  &  &  &  & 1 &\\
      & 1 & \varepsilon & 1 &  &  &  &  &  & 1 &  &\\
      &  & 1 & \varepsilon & 1 &  &  &  & 1 &  &  &\\
      &  &  & 1 & \varepsilon & 1 &  & 1 &  &  &  &\\
     1 &  &  &  & 1 & \varepsilon & 1 &  &  &  