Ab Initio Studies of Systems Containing Actinides Using Relativistic Effective Core Potentials

Book excerpt: Abstract: The major part of this dissertation deals with quantum chemical calculations on atoms, molecules and clusters containing actinide elements. The calculations for these systems are challenging because: (1) correlation, relativistic and spin-orbit (SO) effects are significant and in many cases they are also strongly coupled; (2) three open shells (5f, 6d and 7s) with different angular quantum numbers are close in energy with substantial differences in radial extent and all are involved in bonding and excitations, not only increasing the size of the calculation but also the multireference character of the electronic states; (3) with many low- as well as high-lying (up to near ultraviolet) electronic states lying a few hundred cm−1 from others arising from the same and different electron configurations, a calculation involving higher-order electron correlation needs to be performed; (4) high core-valence correlation is present in many cases among 5s-5f and 6s-6d shells; and, finally (5) the coupling scheme is highly complicated. In spite of the complexities inherent in studying the systems containing actinides, significant developments in theoretical models have resulted in the successful prediction of the electronic properties of systems containing actinides ranging from small systems like UH to large systems like Pu(CH)2. This work is another attempt to apply relativistic ab intio methodology to the study of the electronic structure of systems containing actinides not only in different regions of spectroscopy but also under different chemical environments. In all the systems studied in this work the relativistic effects, correlation effects and spin-orbit effects are strongly coupled. As a result our multireference configuration interaction method (MRCI) is an ideal choice to study these systems. In this work we have analyzed the electronic structure of systems like UO, UO2, UO22, ThO and Cs2UO2Cl4 among many others for which significant amounts of experimental data are present. In addition we have presented theoretical studies of systems like ThO and UO for which experimental work is currently in progress and our work has been used to guide the search for the transitions of interest. Our calculated results are in good agreement with the experimentally obtained results, when available.