Electronic structures of NbGe (n = 1-3) clusters from multiconfigurational CASPT2 and density matrix renormalization group-CASPT2 calculations.

Density functional theory and multiconfigurational CASPT2 and density matrix renormalization group DMRG-CASPT2 have been employed to study the low-lying states of NbGe (n = 1-3) clusters. With the DMRG-CASPT2 method, the active spaces are extended to a size of 20 orbitals. For most of the states, the CASPT2 relative energies are comparable with the DMRG-CASPT2 results. The leading configuration, bond distances, vibrational frequencies, and relative energies of the low-lying states of these clusters were calculated. The ground states of these clusters were computed to be Δ, Φ, and Φ of NbGe ; A , B , and B of cyclic-NbGe ; and A', 1 A″ and 1 A'' ( E), and A″ of tetrahedral-NbGe isomers. For NbGe cluster, our calculations proposed that the ∑ is almost degenerate with the Φ with the CASPT2 and DMRG-CASPT2 relative energies of 0.05 and 0.06 eV. The adiabatic detachment energies of NbGe (n = 1-3) clusters were estimated to be 1.46, 1.55, and 2.18 eV by the CASPT2 method. The relevant detachment energies of the anionic ground state and the ionization energies of the neutral ground states are evaluated at the CASPT2 level.