Calculation of harmonic and anharmonic vibrational wavenumbers for triatomic uranium compounds XUY

The performance of nine different variants of density functional theory (DFT) for the calculation of the vibrational frequencies of the triatomic compounds UO22+, NUN, NUO+ and CUO is investigated and compared with the predictions obtained with the CASPT2 method [Chem. Phys. Lett. 331 (2000) 229]. Vibrational anharmonicity is calculated for UO22+ and for CUO and is shown to be small for these systems. A detailed comparison of experimental data obtained in rare-gas matrices with the DFT predictions shows that while the performance of hybrid DFT is uneven, the more elementary GGA versions such as PW91 perform extremely well, as the estimated unsigned (signed) average errors are only 11 (-2) cm(-1) for the five observed IR bands. Overall, the DFT/GGA results for the closed-shell compounds considered here are at least as good as those yielded by the CASPT2 method, which is computationally much more demanding. Calculated IR intensities are in at least semi-quantitative agreement with experiment. We suggest that the vibrational data becoming available for actinide compounds could and should be used to obtain more reliable parameters in hybrid versions of DFT. The calculated harmonic stretching force constants are shown to depend critically on the U-X bond length, but they are almost insensitive to the method used for their calculation, at a given distance. Observed vibrational spectra can therefore be used to infer reliable bond lengths. (C) 2004 Elsevier B.V. All rights reserved.