The first (β) and second (γ) hyperpolarizabilities of BC5 clusters were investigated through the PM6, PM3, and MNDO-d semiempirical Hamiltonians. The Im2, Imm2, P3m1, and P phases that are pointed out in literature as likely candidates of the BC5 superhard diamondlike real structure were investigated. In this way, the stability analysis of the BC5 clusters was performed. Moreover, insights into the nonlinear mechanisms were obtained via Resonating Valence Bond (RVB) theory. The semiempirical methodology was evaluated in relation to the β and γ hyperpolarizabilities by considering fully characterized nonlinear organic molecules reported in the literature. The high values of the dynamic β and γ hyperpolarizabilities exhibited by the BC5 phases demonstrate that these clusters present potential applications as nonlinear optical media.

Cálculos DFT em modelos de cluster de cupratos (LaBa2Cu3O6.7, La1.85Sr0.15CuO4, YBa2Cu3O7, TlBa2Ca2Cu3O8.78, HgBa2Ca2Cu3O8.27), de sistemas metálicos (Nb3Ge, MgB2) e do pnictido LaO0.92F0.08FeAs indicaram a ocorrência da ressonância não-sincronizada no estado supercondutor, como prevê a teoria RVB (resonating valence bond) de Pauling. Para os cupratos, a ressonância não-sincronizada envolve transferência de elétron entre átomos de Cu, acompanhada pela redução da carga dos átomos La, Sr, Y e Ca. Para o MgB2, a transferência de elétron ocorre na camada do Mg, enquanto a do B comporta-se como reservatório de cargas. Para o Nb3Ge, a ressonância ocorre entre átomos de Ge, que devem ser os responsáveis pela transferência de carga. Para o LaO0.92F0.08FeAs, ambas as camadas de La-O e Fe-As estão envolvidas no mecanismo da supercondutividade. A observação de ressonâncias não-sincronizadas nestes sistemas fornece uma evidência em favor da RVB como uma teoria apropriada para a supercondutividade em altas temperaturas (high-T C).

DFT calculations performed on different cluster models of cuprates (LaBa2Cu3O6.7, La1.85Sr0.15CuO4, YBa2Cu3O7, TlBa2Ca2Cu3O8.78, HgBa2Ca2Cu3O8.27), metallic systems (Nb3Ge, MgB2) and the pnictide LaO0.92F0.08FeAs made evident the occurrence of unsynchronized resonance of covalent bonds in the superconducting state, as predicted by Pauling´s resonating valence bond (RVB) theory. For cuprates, the unsynchronized resonance involves electron transfer between Cu atoms accompanied by a decrease in the charge of the La, Sr, Y and Ca atoms. For MgB2, electron transfer occurs in the Mg layer, while the B layer behaves as charge reservoir. For Nb3Ge, unsynchronized resonance occurs among the Ge atoms, which should be responsible for charge transfer. For LaO0.92F0.08FeAs, the results suggest that both La-O and Fe-As layers are involved in the mechanism of superconductivity. The identification of unsynchronized resonances in these systems provides evidence which supports RVB as a suitable theory for high-temperature superconductivity (high-T C).