On the structure of the lowest spin states of [formula omitted]. Hybrid DFT vs. benchmark CASSCF-CASPT2 studies

Abstract

The three lowest spin states (S = 0,1,2) of Li13+ are investigated using density functional theory (DFT) and benchmark CASSCF and CASPT2 with a couple of Dunning's cc-pVnZ basis sets. Full CASSCF(12,12)/cc-pVDZ geometry optimizations were done using optimized PBE0 structures. Since 12 electrons in 12 active orbitals span the complete active valence space, the present CASPT2/cc-pVTZ results provide benchmarks for this cationic cluster. The CASPT2(12,12)/cc-pVTZ lowest energy structures are a centered-squared antiprism singlet (S = 0) and a quintet (S = 2) icosahedron lying 3.4 kcal/mol above the former. The lowest triplet lies 5.6 kcal/mol above the singlet ground state and all the spin states have a rather strong multireference character. The dynamical behavior of this floppy cluster actually precludes the identification of a dominant structure for Li13+. Prediction of molecular properties is found to be very sensitive both to the electronic correlation treatment and the basis set used.