Density Functional Theory versus Complete Active Space Self-Consistent Field Investigation of the Half-Metallic Character of Graphite-Like and Amorphous Carbon Nanoparticles

Model carbon nanoparticles representative
of the graphite-like
and amorphous domains of active carbon are investigated with density
functional theory (DFT) and complete active space self-consistent
field (CASSCF) methods. Cyclic carbon clusters containing conjugated
carbene groups are found to undergo Jahn–Teller distortion.
More importantly, the half-metallicity, that is, the equal or similar
stability of various spin states, previously suggested by DFT calculations
for both types of nanosized clusters is confirmed by CASSCF calculations.
Furthermore, the model carbon clusters are found to possess a multiconfigurational
electronic structure dominated by high-spin configurations. When compared
to CASSCF results, the single-reference DFT predicts proper electronic
structures, characterized by antiferromagnetically coupled electron
pairs, at the expense of spin contamination as a reflection of the
multiconfigurational character. In fact, spin contamination, which
is normally viewed as an error, does not corrupt the energetics of
the half-metallic systems and therefore does not preclude the applicability
of DFT to such systems.