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
The work involves determination of the electronic structure of
molecules, including relativistic effects necessary for heavy
elements. Most calculations are very challenging and well suited
to the Cray T3E. 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 J90s and the T3E.
Application work: Among many topics, we are studying the
speciation of aqueous uranium (VI) carbonates 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
This is the second year of this project. For benchmark purposes,
the very rigorous relativistic models continue to be used to provide
data for small uranium carbonates and also to design basis sets.
This work has been performed on the J90s and the T3E. 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.
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A large component of the work on the T3E has been spin-orbit
configuration interaction (SO-CI) calculations on various actinide
ions. Experimental results indicate that aqueous lanthanides and
actinides may be physically separated under conditions of high
concentration of the soft donor chloride ion. A fundamental understanding
of this process would assist in suggesting better choices and
design of soft donor ligands for more selective and faster separation
of these classes of elements. We are beginning to acquire pertinent
information on this process by generating large spin-orbit electronic
structure models of the AmCl2+ and EuCl2+
molecules and are looking into the differences in bonding characteristics.
The new parallel SO-CI code is now scaling very well and is functioning
on the NERSC Cray T3E as well as the IBM SP machines at ANL and
PNNL. The improved performance is due to numerous optimizations
and use of the Global Array and ChemIO tools. A spin-orbit CI
calculation comprises several steps. The input is the molecular
orbital integrals arising from a self-consistent field calculation.
Then the CI space must be defined. This step can require some
skill in order to provide a balanced and accurate description
of the electronic states of interest in a computationally tractable
expansion. Work on this project has extended the size of viable
expansions by over an order of magnitude. Next the Hamiltonian
matrix must be generated and stored on disk. The structure and
sparsity of this matrix is illustrated in the figure, and must
be accounted for in efficient implementations. Finally, Davidson's
method is used to determine iteratively the eigenvalues of interest,
usually about the lowest dozen states.
Significance
Most radioactive waste involves actinides, and their large atomic
number implies that relativistic effects have important chemical
consequences. Our implementation and application of relativistic
quantum chemical methods on massively parallel computers will
provide capabilities for modeling heavy-element compounds similar
to those currently available for light-element compounds. The
theoretical and computational methodology so developed will supplement
current, very expensive experimental studies of the actinides
and lanthanides. This will allow limited experimental data to
be extrapolated to many other regimes of interest.
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