ORIGINAL RESEARCH Theoretical study of the Pu and Am dicarbide molecules Peter Poga ´ny • Attila Kova ´cs • De ´nes Szieberth • Rudy J. M. Konings Received: 22 March 2012 / Accepted: 23 May 2012 / Published online: 9 June 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The electronic structure and ground-state molecular properties of Pu and Am dicarbides were investigated in this study by relativistic multireference calculations using CASSCF/CASPT2 theory. The elec- tronic ground states in both the molecules correspond to a symmetrical triangular structure, in which the actinide atom is connected to a C 2 moiety. The other experimentally relevant structure, the symmetrical linear arrangement, is higher than 100 kJ/mol in energy. The bonding character- istics in the two structures were analysed on the basis of the valence molecular orbitals. Besides the usual two-electron bonds, a few one-electron bonding orbitals were also identified. We evaluated the electronic spectra with com- plete active space state interaction (CASSI) calculations taking into account spin–orbit coupling. The infrared and Raman spectral characteristics were obtained using DFT calculations. Assessment of the present results with the previously reported data on Th and U dicarbides provides information on the change of various molecular parameters in the actinide row. Keywords Actinide dicarbides  Multireference calculations  DFT  Bonding  Electronic spectra Introduction The major importance of actinide (An) carbides lies in their potential application as advanced nuclear fuel. It raises numerous technological and safety issues including the thermodynamical behaviour of the species in the vapour phase. Unfortunately, information on gaseous actinide carbides is scarce. Some AnC n species (An = U[1–5] and Th [4, 6–9]; n = 1–6) have been detected in the vapours above solid carbides and metal alloys in graphite systems. In these studies, partial pressures of the carbide molecules have been measured by mass spectrometry, and from these results, thermodynamical data have been calculated. Recently, some infra red (IR) absorption bands of UC and UC 2 have been measured using the matrix-isolation tech- nique [10, 11]. In addition to the above experimental investigations, a few theoretical studies have been performed on small carbide molecules. The early discrete variational Xa cal- culations on UC 2 [12] and the first DFT calculations using relativistic effective core potentials (RECP) on UC 2 [13] had several deficiencies. (The related DFT study on PuC and PuC 2 [14] will be discussed in the present article.) Thus, the correct electronic ground states of the uranium carbides have been elucidated only recently using advanced relativistic multireference calculations [10, 11]. The molecular structures of the less problematic closed- shell thorium carbides (ThC 2 and ThC 4 ) have been reported from MP2/RECP calculations [15]. Recently, we have performed a detailed analysis of the five possible dicarbide structures of ThC 2 and UC 2 [16] using relativistic Electronic supplementary material The online version of this article (doi:10.1007/s11224-012-0075-2) contains supplementary material, which is available to authorized users. P. Poga ´ny  A. Kova ´cs (&)  R. J. M. Konings Institute for Transuranium Elements, European Commission, Joint Research Centre, P.O. Box 2340, 76125 Karlsruhe, Germany e-mail: attila.kovacs@ec.europa.eu A. Kova ´cs  D. Szieberth Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gelle ´rt te ´r 4, 1111 Budapest, Hungary 123 Struct Chem (2012) 23:1281–1289 DOI 10.1007/s11224-012-0075-2