The optimization of molecular geometries for systems with multiconfigurational electronic structures can be performed using the recently implemented fully internally contracted analytical gradients for the second-order complete active space SCF method (CASPT2). We demonstrate the performance of this method for two types of complexes. The first family of complexes explored presents with a characteristic geometry change for systems ranging in size from 13 to 61 atoms. Specifically, the metal-ligand bond distance in Fe(II) spin-crossover complexes is elongated in the high spin state compared to the low spin state. The quality of the optimized geometry and its impact on the relative energies of the high-spin and low-spin state are assessed. Next, we move beyond structural comparisons to study the vibrational spectroscopy of chromium-chromium metal-metal bonds which present prototypical examples of multiconfigurational electronic structures. We explore three complexes and show significant improvement over DFT geometries, although the choice of CASPT2 zeroth-order Hamiltonian is important. We calculate the Raman active metal-metal and metal-ligand stretching modes, making comparisons with isotope labeling experiments. The effect of anharmonicity is also explored by normal mode sampling.

Understanding spin-crossover and vibrational spectra with multireference methods

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