Oak Ridge National Laboratory

Physics Division

Physics Division Seminars

Physics Division Seminars bring us speakers on a variety of physics related subjects. Usually these are held in the Building 6008 large Conference Room, at 3:00 pm on the chosen day, but times and locations may vary. For more information, contact our seminar chairman,

Alfredo Galindo-Uribarri
Tel (Office): (865) 574-6124  (FAX): (865) 574-1268

Thu., July 17, 2003, at 3:00 p.m.

New Alternatives for Accurate Electronic Structure Calculations for Molecular Systems

Piotr Piecuch, Michigan State University
Building 6008, Conference Room

In this talk, I will focus on the "holy grail" of the electronic structure theory, which is the development of accurate and affordable ab initio methods that can provide a highly accurate description of molecular systems. An emphasis will be placed on the coupled-cluster theory of Coester, Kummel, and Cizek and its recent extensions to quasi-degenerate and excited states. First, I will discuss a new many-body formalism, termed the method of moments of coupled-cluster equations (MMCC), which is based on rigorous mathematical relationships that define the many-body structure of the differences between the exact, full configuration interaction (full CI), and coupled-cluster (CC) or equation-of-motion coupled-cluster (EOMCC) energies. I will show that the MMCC formalism enables us to renormalize the existing standard "bkack-box" single-reference CC and EOMCC methods, such as CCSD(T) or EOMCCSD, so that they can be used to describe quasi-degenerate electronic states, chemical bond breaking, and excited states of molecular systems dominated by two-electron and other many-electron transitions without invoking advanced multi-reference concepts. Next, I will discuss new possibilities offered by the extended CC theory of Arponen and Bishop and Piecuch and Bartlett which, when combined with the MMCC formalism, enables us to describe strongly correlated electronic states, which emerge when multiple chemical bonds are broken, with the ease of the basic CC singles and doubles (CCSD) calculation. Finally, if time permits, I will discuss an interesting issue of representing the exact wave functions of many-electron systems (and other many-fermion systems with pairwise interactions) by exponential cluster expansions employing two-body operators only.