CASPT2 molecular geometries of Fe(II) spin-crossover complexes

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Inorganic Chemistry

Background:

• Spin-crossover (SCO) complexes are crucial in materials science.
• Accurate geometry optimization is essential for understanding SCO behavior.
• Wave function-based methods offer high accuracy but can be computationally expensive.

Purpose of the Study:

• To evaluate the efficiency and accuracy of fully internally contracted (FIC)-CASPT2 analytical gradients for SCO complex geometry optimization.
• To investigate the impact of active space and basis set choices on geometry predictions.
• To compare CASPT2 results with Density Functional Theory (DFT) and experimental data.

Main Methods:

• Geometry optimizations using FIC-CASPT2 analytical gradients.
• Testing on a series of Fe(II) complexes (13-61 atoms).
• Exploration of various active space and basis set combinations.

Main Results:

• FIC-CASPT2 provides reliable molecular geometries for metal complexes in reasonable computation times.
• Good agreement between CASPT2 and DFT geometries was observed, with notable differences for smaller complexes with strong field ligands.
• Moderate basis sets yielded geometries comparable to DFT and experimental data for larger complexes.

Conclusions:

• Wave function-based theory is a viable approach for optimizing metal complex geometries.
• The (10e,12o) active space is recommended for efficient convergence to minimum energy structures.
• This method allows simultaneous treatment of molecular geometry and electronic structure at the same theoretical level.