Abstract
Density functional theory (DFT) including van der Waals weak interaction in conjunction with the so called rotational invariant DFT+U, where U is the Hubbard interaction of the iron site, is used to show that the fluorinated spin crossover Fe(phen)2(NCS)2 molecule whether in the gas phase or adsorbed on Cu(001) surface switches from the original low spin state to the high spin state. The calculated minimal energy path by means of both the nudged elastic band method and the constrained minimization method is found to be smaller for the fluorinated molecule. Using Bader electron density analysis and a point charge model, this inversion of the spin crossover is explained in terms of electron doping of the Fe-octahedron cage which led to an increase of the Fe–N bond lengths and the distortion of the Fe(II) octahedron. Consequently, the ligand-field splitting is drastically reduced, making the high-spin ground state more stable than the low-spin state. The calculated scanning tunneling microscopy (STM) images in the Tersoff–Hamann approximation show a clear distinction between the fluorinated and the unfluorinated molecule. This theoretical prediction is awaiting future STM experimental confirmation.
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