On the three lowest spin states of Na13+. Hybrid DFT and Benchmark CASSCF(12,12)+CASPT2 studies
• Emiliano Alanís-Manzano,
• Alejandro Ramírez-Solís
Emiliano Alanís-Manzano
Universidad Nacional Autonoma de Mexico Instituto de Ciencias Fisicas

Corresponding Author:emiliano@icf.unam.mx

Author Profile
Alejandro Ramírez-Solís
Author Profile

## Abstract

The three lowest spin states (S=0,1,2) of twelve representative Na13+ isomers have been studied using both, KS-DFT via three hybrid density functionals, and benchmark multireference CASSCF and CASPT2 methods with a couple of Dunning’s correlation consistent basis sets. CASSCF(12,12) geometry optimizations were carried out. Since 12 electrons in 12 active orbitals span the chemically-significant complete valence space, the results of the present study provide benchmarks for Na13+. The CASPT2(12,12)/cc-pVTZ* lowest energy structures are three nearly degenerate singlets (S=0): an isomer formed from two pentagonal bipyramids fused together (PBPb), a capped centered-squared antiprism [CSAP-(1,3)] and an optimum tetrahedral OPTET(II) structure, the last two lying 0.88 and 1.63 kcal/mol above the first, respectively. The lowest triplet (S=1) and quintet (S=2) states lie 4.33 and 3.77 kcal/mol above the singlet global minimum, respectively. The latter is a deformed icosahedron while the former is a CSAP-(1,3). The flatness of the potential energy surface of this cluster suggests a rather strong dynamical character at finite temperature. Prediction of the lowest energy structures and electronic properties is crucially sensitive both to non-dynamical and dynamical electron correlation treatment. The CASPT2 vertical ionization energy is 3.66 eV, in excellent agreement with the $3.6 \pm 0.1$ eV experimental figure. All the isomers are found to have a strong multireference character, thus making Kohn-Sham density functional theory fundamentally inappropriate for these systems. Only large multiconfigurational complete active space self-consistent field (CASSCF) wavefunctions provide a reliable zeroth-order description; then the dynamic correlation effects must be properly taken into account for a truly accurate account of the structural and energetic features of alkali-metal clusters.
14 May 2022Submitted to International Journal of Quantum Chemistry
19 May 2022Submission Checks Completed
19 May 2022Assigned to Editor
19 May 2022Reviewer(s) Assigned
05 Jul 2022Review(s) Completed, Editorial Evaluation Pending
05 Jul 2022Editorial Decision: Revise Minor