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Research at the CCC
The Center for Computational Chemistry (previously known as the Center for Computational Quantum Chemistry)
at the University of Georgia seeks to develop theoretical and
computational methods through mathematical models for describing and understanding the movement and
function of electrons in molecules and to apply the theoretical methods to significant problems of
broad chemical interest.
Some of the theoectical methods under development include the multiconfiguration self-consistent-field
(MCSCF), configuration interaction, coupled-cluster and Brueckner methods, and associated analytic
gradient techniques. Additional theoretical work involves density-function theory, the evaluation
of electron repulsion integrals, and the devlopment of methods with explicit dependence
interelectronic coordinates.
Currently applications to several areas of chemistry are of special concern:
Professor Schaefer's early work resulted in the formulation (1971) of a ''first order'' wave function
for the oxygen molecule. The ideas set out there have played a role in the development of current
understandings of the nature of valence electron correlation. This research was followed by the
development of a theory of self-consistent electron pairs (1976) and generalized direct configuration
interaction methods (1978). Also influential has been the formulation of the loop-driven (1979) and
shape-driven (1982) graphical unitary group approaches. These new methods illuminate large numbers of
previously unrecognized relationships between Hamiltonian matrix elements and provide a facile path
via the two-particle reduced density matrix to analytic gradients (1980) and second derivatives (1984)
for explicitly correlated wave functions.
In 1987 it was possible to formulate the analytic gradient technique for the single and double
excitation coupled cluster (CCSD) approach to the correlation problem. In 1989 the full coupled
cluster method including all connected triple excitations (CCSDT) was developed and tested for a
variety of molecular systems. Theoretical work in 1994 showed how to variationally treat triple
and quadruple excitations in a highly compact and efficient manner. Open-shell coupled cluster and
Brueckner methods (1997, 1998) have been the focus of much recent research. Our latest
methodological work (2000) has involved (a) an assessment of the convergence properties of
perturbation theory and (b) the use of explicit interelectronic coordinates in quantum chemistry.
Such advances have begun to contribute to truly reliable quantum mechanical explorations of
potential energy hypersurfaces, upon which the course of chemical reactions intimately depends.
The scope and caliber of the Center for Computational Quantum Chemistry's applications of
electronic structure theory to chemistry are unusual. The work is often marked by (a) the use
of genuine state-of-the-art theory; (b) a significant degree of thoughtfulness and thoroughness;
and (c) a healthy familiarity with the experiments with which the theoretical research is
intended to interact. The most visible result of this approach has been the reversal of the
conclusions of a number of distinguished experimentalists. Subsequent experimental studies have
verified controversial predictions concerning the structure and singlet-triplet separation of
methylene, the quadrupole moment of ozone, the activation energy for D + HF exchange, and the
vibrational spectrum of benzyne, to cite just a few. Equally important has been the manner in
which work in the Schaefer group has guided experimentalists in many new and fruitful
directions. A recognized early example [Nature 278, 507 (1979)] was the identification of the
spectrum of triplet acetylene.
Funding for this research is provided by the National Science Foundation, the Department of
Energy, and the North Atlantic Treaty Organization.
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