The Hückel approximation is used to determine the energies and shapes of the π π molecular orbitals in conjugated systems. Within the Hückel approximation, the covalent bonding in these hydrocarbones can be separated into two independent "frameworks": the σ σ -bonding framework and the the σ σ -bonding framework. The wavefunctions used to describe the bonding orbitals in each framework results from different combinations of atomic orbitals. The method limits itself to addressing conjugated hydrocarbons and specifically only π π electron molecular orbitals are included because these determine the general properties of these molecules; the sigma electrons are ignored. This is referred to as sigma-pi separability and is justified by the orthogonality of σ σ and π π orbitals in planar molecules. For this reason, the Hückel method is limited to planar systems. Hückel approximation assumes that the electrons in the π π bonds “feel” an electrostatic potential due to the entire σ σ -bonding framework in the molecule (i.e. it focuses only on the formation of π π bonds, given that the σ σ bonding framework has already been formed).