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.本研究采用密度泛函理论（DFT）与完全活性空间自洽场（CASSCF）方法，对具有类石墨域与无定形域特征的模型活性炭碳纳米颗粒展开探究。研究发现，带有共轭卡宾基团的环状碳团簇会发生姜-泰勒畸变。更为关键的是，此前DFT计算针对两类纳米团簇所提出的半金属性——即不同自旋态具有相等或相近的稳定性——已通过CASSCF计算得到验证。此外，该模型碳团簇被发现拥有以高自旋组态为主导的多组态电子结构。与CASSCF结果相比，单参考密度泛函理论虽能正确预测以反铁磁耦合电子对为特征的电子结构，却存在自旋污染问题，这正是其多组态特性的体现。实际上，通常被视为误差来源的自旋污染，并不会破坏半金属体系的能量学性质，因此并不妨碍密度泛函理论在这类体系中的应用。