R. J. Harrison, J. A. Nichols, R. A. Kendall, D. A. Dixon, T. H. Dunning, Jr., J. Nieplocha, and G. I. Fann, Environmental Molecular Sciences Laboratory,
Pacific Northwest National Laboratory
R. Shepard, A. F. Wagner, R. Stevens, J. L. Tilson, and M. Minkoff,
Argonne National Laboratory
C. W. McCurdy and A. T. Wong, NERSC, Lawrence Berkeley National Laboratory
R. M. Pitzer, The Ohio State University
D. E. Bernholdt, Syracuse University
W. Ermler, Stevens Institute of Technology
K. G. Dyall, Eloret, NASA Ames Research Center

Research Objectives

We aim to develop and apply the methods of relativistic quantum chemistry to assist in the understanding and prediction of the chemistry of actinide and lanthanide compounds.

Computational Approach

	A model for uranyl in aqueous solution: (UO2)2+ (H2O)15.

The work involves determination of the electronic structure of molecules including relativistic effects necessary for heavy elements. There are four major categories of activities:

Benchmarking of methods: Detailed and systematic comparison of various theoretical approaches with each other and with experiment. Few such studies are available for rigorous relativistic methods, and still fewer for systems containing actinides. This work uses the Cray J90s and the T3E.

Application work: Among many topics, we are studying the speciation of aqueous uranium (VI) with various ligands, and the electronic spectra of several systems, including AmCl²⁺. A detailed understanding of the actinide-carbonate-water system is essential to modeling the fate and transport of actinides in the environment. This work uses the T3E.

Method and computer program development: Existing programs are being parallelized for the T3E and extended to enable calculations on larger molecules at higher levels of accuracy.

Computer science: Extensions of global arrays, parallel I/O, new linear algebra, metacomputing, and prototyping of new parallel programming tools for the T3E and other parallel computers.

Accomplishments

In order to determine even qualitatively correct electronic spectra for heavy metals, especially for actinides, the effects of both electron correlation and the spin-orbit interaction must be taken into account. A large component of the work on the T3E has been spin-orbit configuration interaction (CI) calculations upon various actinide ions. Much effort has been devoted to developing and understanding accurate descriptions of the electronic spectra of various actinide and lanthanide ions. This is very challenging and has required development of new relativistic effective core potentials. New all-electron relativistic approximations that have been incorporated into NWChem are being tested by comparison with all-electron Dirac-Fock calculations. These new methods also require development of multiple new basis sets (up to three per atom).

Significance

Most radioactive waste involves actinides, and their large atomic number implies that relativistic effects have important chemical consequences. Our implementation of relativistic quantum chemical methods on MPP computers will provide capabilities for modeling heavy-element compounds similar to those currently available for light-element compounds. This methodology will supplement expensive experimental studies of the actinides and lanthanides. This will allow limited experimental data to be extrapolated to many other regimes of interest.

Publications

K. G. Dyall and E. van Lenthe, "Relativistic regular approximations revisited: An infinite-order regular approximation," J. Chem. Phys. 111, 1366 (1999).

S. Yabushita, Z. Zhang, and R. M. Pitzer, "Spin-orbit configuration interaction using the graphical unitary group approach and relativistic core potential and spin-orbit operators," J. Phys. Chem. A 103, 5791 (1999).

Z. Zhang and R. M. Pitzer, "Application of relativistic quantum chemistry to the electronic energy levels of the uranyl ion," J. Phys. Chem. A 103, 6880 (1999).