DOI: 10.1002/chem.200601244 A Comparative Relativistic DFT and Ab Initio Study on the Structure and Thermodynamics of the Oxofluorides of Uranium(IV), (V) and (VI) Grigory A. Shamov, Georg Schreckenbach,* and Thach N. Vo [a] Introduction Theoretical ab initio quantum chemical and density func- tional (DFT) methods have proven to be a very helpful tool for main group and transition-metal compounds. [1] They could also be capable of improving our understanding of the various aspects of the chemistry of the f elements, especially actinides where experiments are often costly and dangerous. However, actinides are difficult objects to study by theoreti- cal methods because of the importance of both relativistic effects and correlation effects for their compounds. [2–4] Since actinide atoms are “big” in terms of computational costs, the computational methods have to be very robust to model systems of chemical interest, yet accurate enough to give sensible results. The accuracy of the DFT calculations can be tested by comparing the available experimental data against calculat- ed values; comparative studies with other, namely wave- function-based methods are also of great interest. For the actinides, various types of experimental data exist: the most abundant are geometries of the classical Werner-type com- plexes from X-ray crystal and EXAFS solution spectroscopy and actinyl group stretching frequencies from IR/Raman spectroscopy. However, for such ionic compounds, the selec- tion of a realistic computational model system is difficult, Abstract: All the possible uranium(VI, V, IV) oxides, fluorides and oxofluor- ides were studied theoretically by using density functional theory (DFT) in the generalised gradient approximation (GGA), and three different relativistic methods (all-electron scalar four com- ponent Dyall RESC method (AE), rel- ativistic small-core ECPs, and zeroth order regular approximation ZORA). In order to test different correlation methods, for the two former relativistic methods hybrid DFT, and, for the AE method, MP2 molecular orbital calcu- lations were performed as well. Single- point AE-CCSD(T) energies were cal- culated on MP2 geometries as well. Energies of the uranium(VI) and (V) oxofluorides dissociation, uranium(VI) fluoride hydrolysis and oxofluoride dis- proportionation were calculated and compared against the available experi- mental thermochemical data. AE- CCSD(T) energies were the closest to the experiment. For GGA DFT meth- ods, all the relativistic methods used yield similar results. For thermochemis- try, the best quantitative agreement with the experimental and CCSD(T) values for both U=O and U  F bond strengths was obtained with hybrid DFT methods, provided that a reliable basis set was used. Both the GGA DFT and MP2 MO methods show overbinding of these bonds; moreover, this overbinding was found to be not uniform but strongly dependent on the coordination environment of the urani- um atom in each case. U=O vibrational frequencies given by hybrid DFT, how- ever, are systematically overestimated, and are better reproduced by GGA DFT; MP2 values usually fall in-be- tween. Reaction enthalpies, U=O fre- quencies and complex geometries given by the PBE, MPBE, BPBE, BLYP and OLYP GGA functionals are quite similar, with OLYP performing slightly better than the others but still not as good as hybrid DFT. The geo- metries of the molecules are found to be influenced by the following factors: the inverse transinfluence (ITI) of the oxygen ligand and, for U V , and U IV , the Jahn–Teller distortion. Keywords: ab initio calculations · density functional calculations · fluorides · relativity · uranium [a] Dr. G.A. Shamov, Prof. G. Schreckenbach, T.N. Vo Department of Chemistry, University of Manitoba Winnipeg, Manitoba, R3T 2N2 (Canada) Fax: (+ 1) 204-474-7608 E-mail : schrecke@cc.umanitoba.ca Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author: Table S1 contain- ing the dimensions of the orbital and density basis sets used in AE calculations; Table S2 containing U=O vibrational frequencies calcu- lated by different GGA functionals with the AE relativistic method; description of the complete basis set extrapolation procedure applied for MP2 calculations; Table S3 containing MP2 correlation and total energies of the molecules; these energies were used in the complete basis set extrapolation procedure. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 4932 – 4947 4932