Iterating this procedure leads to a [*RG flow*]{} in the parameter space of the model and the hope is to find a fixed point of this flow behaviour. Such a fixed point Hamiltonian is helpful to determine the universal behaviour of the physical model.\
As explained in the introduction, the boundary conditions of a block within the quantum system are essential for the calculation of a [*RG step*]{}, which is defined as one application of the RGT. In fact the different boundary conditions represent the correlations in the quantum chain between adjacent blocks to which we refer in the following as [*system blocks*]{}. To provide the opportunity to choose those boundary conditions, which result in the most accurate representation of an isolated block, the fundamental idea of DMRG is to embed the system block into a “bigger” block, called [*superblock*]{}. This nomenclature as well as the term system block is due to the original work of S.R. White [@wh93]. In some sense this simulates the environment represented by the surrounding spin sites and effectively smoothes out the sharp effects of the boundary conditions, as depicted in figure \[superblock\].\

To construct a working approach out of this overall picture we are 