Geometric and electronic structures of FeB clusters (n = 1-3): insights from advanced computational methods.

CONTEXT

Boron-doped iron clusters are extensively studied for their potential in materials science. Despite several quantum calculations with DFT and MRCI methods, a comprehensive understanding of the geometric and electronic structures of small FeB clusters (n = 1-3) is still lacking. This work provides new insights into ground and low-lying excited states, detachment energies, and ionization energies of these clusters using DFT and multireference CASPT2, RASPT2, and DMRG-CASPT2 computational methods. Key findings reveal Σ, Σ, and Σ as ground states for FeB, and cyclic-FeB isomers (B, B, B) as the most stable for FeB clusters. For FeB clusters, anionic species have a tetrahedral geometry, while neutral and cationic species favor rhombic structures. Detachment energies of the anionic ground states increase progressively from FeB to cyclic-FeB, and further to the tetrahedral-FeB isomer, which correlates with the number of boron atoms bonded to the iron atom. The vibrational progression in transitions within tetrahedral-FeB is more prominent than in FeB clusters and cyclic-FeB isomers. The ionization energies of neutral ground states rise from FeB clusters to rhombic-FeB and cyclic-FeB isomers.

METHODS

The geometry optimization and vibrational frequency calculations for the electronic states of FeB and FeB clusters were conducted using density functional theory (DFT) with the BP86 and MN15 functionals and the def2-QZVP basis set, implemented in ORCA 5.0. Franck-Condon factor simulations were performed using the ezSpectra suite, based on DFT-derived geometries and vibrational normal modes. Multireference RASPT2 and CASPT2 calculations utilized OpenMolcas, while DMRG-CASPT2 calculations employed ChemPS2 interfaced to OpenMolcas. The aug-cc-pwCVQZ-DK basis set was applied to iron, and aug-cc-pVQZ-DK to boron. The 1 s, 2 s, and 2p orbitals of iron and the 1 s orbital of boron were frozen in the second-order perturbation calculations. IPEA and imaginary shift parameters were set to 0.25 and 0.10, respectively. To achieve high accuracy, the DMRG-CASPT2 active spaces were expanded to 22 orbitals for FeB and FeB, and 23 orbitals for FeB.