  Cohesive energies, energy-volume equations of states EOS, electron
localization ELF maps, elastic constants, and band structures are reported for
original octacarbon C8 three dimensional 3D and two-dimensional 2D chemical
systems based on density functional theory calculations. Specifically,
tetragonal C8 is identified cohesive with hardness close to experimentally
identified cubic Ia-3 C8; both exhibiting comparable hardness to diamond. Also,
isoelectronic and isostructural B4N4 is calculated with a slightly lower
hardness due to the ionocovalent B-N bonding and a bandgap with the same
magnitude as Diamond. 2D-C8 on the other side is proposed with interpenetrating
two carbon hexagonal substructures, identified from energy calculations as
stable in a ferromagnetic ground state. Critical pressure for the collapse of
magnetization PC=12 GPa let assign a soft ferromagnetic behavior alike Invar
alloys. Electronic band structures analyses identify specific bands
differentiating magnetic carbon substructure (C1) from nonmagnetic
semi-conducting honeycomb-like C26 layers. These observations let propose spin
chemistry perspectives once such multilayered carbon 2D compounds are grown as
stand-alone or on selected substrates as thin or thick films
