A CASPT2 calculation of the lowest excited states of H2Fe(CO)4

CASPT2 calculations based on CASSCF reference wavefunctions, using large atomic natural orbital basis sets, are reported for the lowest excited states of H2Fe(CO)4. The excitation energies obtained with this accurate method are compared to the values from multireference contracted CI calculations based on a unique CASSCF wavefunction. This computational strategy is usually employed for the calculation of the one-electron properties, spin-orbit interaction or costly potential energy surfaces describing the photochemistry of organometallic molecules. The relative order of the excited states is not affected by this more refined treatment and the largest excitation energy differences are of the order of 0.25 eV for the lowest excited states. However, the computational strategy seems to have a dramatic influence on the results obtained for the highest excited states pointing out the limit of less accurate methods. The theoretical spectra obtained for this family of molecules must be compared with poorly resolved experimental spectra.