Towards an accurate and computationally-efficient modelling of Fe(II)-based spin crossover materials

Area of Science:

• Computational materials science
• Solid-state chemistry
• Quantum chemistry

Background:

• Density Functional Theory (DFT) + U is a promising method for molecular materials.
• Accurate results depend on careful parametrization of the U-term.
• Spin crossover (SCO) compounds exhibit distinct high-spin (HS) and low-spin (LS) states.

Purpose of the Study:

• To benchmark the Hubbard-like U-term for Fe(ii)N6-based SCO compounds.
• To establish a reliable U-value for accurate energetic calculations in the solid state.
• To quantify the impact of intermolecular interactions on SCO behavior.

Main Methods:

• Benchmarking the Hubbard-like U-term within DFT + U.
• Using experimental data (T1/2, ΔS, ΔH) to estimate electronic enthalpy difference (ΔHelec).
• Comparing DFT+U results for gas and solid phases to assess intermolecular effects.

Main Results:

• Parametrized U-values ranged from 2.37 eV to 2.97 eV, averaging 2.65 eV.
• The average U value yielded a mean absolute error (MAE) of 4.3 kJ mol⁻¹ for ΔHelec.
• Intermolecular interactions influenced HS/LS state stability by ~5 kJ mol⁻¹, with variable sign.

Conclusions:

• An average U value of 2.65 eV provides a good starting point for DFT+U studies on SCO compounds.
• This approach enhances energetic accuracy for solid-state systems compared to bare DFT.
• Findings facilitate future research on SCO crystalline phases and surface adsorption phenomena.