Assessment of Simultaneous Global Optimization of Geometry and Total Spin of Small Iron Clusters

In this work, simultaneous global
optimization of geometry and
total spin of small iron clusters Fen (3
≤ n ≤ 40) is assessed using Simulated
Annealing (SA) simulations with forces calculated at the DFT-based
Tight-Binding (DFTB) level of theory. In order to optimize the total
spin, the occupancies of α and β densities were allowed
to relax at each SA step, resulting in a continuous variation of the
total spin along the trajectory. The behavior and performance of the
procedure were investigated running two series of 10 independent “long”
molecular dynamics simulations and one series of 100 independent “short”
simulations. Cluster structures optimized with the assessed methodology
reproduced geometries and magnetic moments reported in a previous
work very well where multiple fixed total spin and geometries of iron
clusters were individually probed, and for some clusters, more stable
global minima were found. Other properties such as binding energies
and second energy differences were also calculated in order to compare
with previously reported theoretical and experimental values. The
few mismatches were found to be driven by quasi degenerated ground
states with different magnetic moments or by states with crossing
free energies at high temperatures.