New Coupled-Cluster Methods for Molecular Potential Energy Surfaces

Piotr Piecuch, Department of Chemistry, Michigan State University

Affiliated SciDAC Researchers:
Professor Mark S. Gordon, Iowa State University and Ames Laboratory;
Dr. Michael W. Schmidt, Ames Laboratory;
Dr. David J. Dean, Oak Ridge National Laboratory;
Dr. So Hirata, Pacific Northwest National Laboratory

Interactions with Other SciDAC Participants:
Dr. Robert J. Harrison, Oak Ridge National Laboratory;
Professor Henry F. Schaefer III, University of Georgia;
Professor Wesley D. Allen, University of Georgia

Summary

This research focuses on the development of new generations of ab initio electronic structure methods and general-purpose computer codes, which can provide an accurate description of chemical bond breaking, reactive pathways, electronic excitations in molecules, and molecular properties and spectra. The new coupled-cluster methods developed in this program will enable precise modeling of elementary chemical reactions that occur in combustion, catalysis, photochemistry, and photobiology using state-of-the-art computer platforms.

Background information. Electronic structure calculations, followed by dynamical and other types of molecular simulations, are recognized as a cornerstone for the understanding and successful modeling of chemical processes and properties that are relevant to combustion, catalysis, photochemistry, and photobiology. Novel ab initio approaches, which can accurately describe the complicated motions of electrons in molecules using the first principles of quantum mechanics, and which can provide reliable results even when the experimental information is not available, are critical for these studies. Of particular significance are the approaches that can precisely predict potential energy surfaces of ground and excited states of molecular systems , since potential energy surfaces determine how atoms and chemical bonds rearrange during chemical reactions. To meet the challenge created by terascale computers and to advance electronic structure theory to a new level of accuracy and applicability, so that reliable calculations for all kinds of molecular systems and electronic states become routine, new ab initio methods, algorithms, and computer codes must be developed.

Principal goals. This research program focuses on the development of new ab initio electronic structure methods and highly efficient general-purpose computer codes, based on coupled-cluster (CC) theory, which can describe chemical bond breaking, reactive pathways, molecular potential energy surfaces and properties, and electronic excitations in molecules. The emphasis is on balancing high accuracy, expected from predictive ab initio methods, with ease of use and relatively low computer cost, so that applications are not limited to small few electron systems. Our new renormalized and active-space coupled-cluster methods, and the method of moments of coupled-cluster equations (MMCC) provide an accurate description of molecular potential energy surfaces and excited states at a fraction of the effort associated with expensive multi-reference configuration interaction (MRCI) calculations. Our CC methods can be applied to problems involving bond breaking and excited states, where the standard “black-box” approaches fail. We and other groups have already applied our new methods and computer codes to systems consisting of up to ~ 30 light atoms and ~ 10 transition metal atoms. Our new multi-reference CC methods have computer costs comparable to MRCI approaches, but they describe bond breaking and excited states more accurately than MRCI methods. Our codes are shared with the community by incorporating them in GAMESS, which is a popular, highly scalable electronic structure package distributed at no cost by Professor M.S. Gordon at Iowa State University and Ames Laboratory. Thus, we provide many scientists with powerful computational tools that have not been available to them before.

The most important accomplishments (September 1, 2001 – January 31, 2004):

1. The incorporation of the standard closed-shell CC approaches (CCSD, CCSD(T), etc.) and the renormalized CCSD(T) methods for single bond breaking and diradicals in the GAMESS package.

2. The incorporation of the standard and novel CC methods for excited states in GAMESS (the equation of motion (EOM) CCSD and renormalized EOMCCSD(T) methods; the latter methods are “black-boxes” which provide ~0.1 eV accuracies for many types of excited states). Our CC codes in GAMESS enable routine calculations for ~100 correlated electrons.

3. The extension of the renormalized (EOM)CC and MMCC methods to multiple bond breaking and challenging types of excited states through the development of the high-order MMCC, quadratic MMCC, CI-corrected MMCC, extended CC , and generalized MMCC methods.

4. Strong evidence has been provided that the virtually exact many-electron wave functions can be represented by cluster expansions employing simple two-electron operators (nothing else seems necessary!).

5. The renormalized (EOM)CC approaches developed by us for molecules proved to be capable of providing an accurate description of ground and excited states of nuclei.

Selected plans for 2004-05 and beyond:

GAMESS will be enhanced by incorporating in it the (EOM)CC methods for molecular properties, transition probabilities, geometry optimizations, and transition-state searches. The efficient codes for the renormalized CC/MMCC methods for multiple bond breaking will be included in GAMESS. Open-shell and multi-reference extensions of the (EOM)CC, MMCC, and renormalized CC methods will be developed and included in GAMESS. An effort will be made to parallelize the CC codes in GAMESS (in collaboration with Professor M.S. Gordon).

Impact of the SciDAC team effort (examples): The SciDAC team approach has greatly enhanced the way our group develops computer codes. SciDAC has enabled us to collaborate with Professor M.S. Gordon and Dr. M.W. Schmidt and to incorporate our CC codes in GAMESS. Dr. R.J. Harrison helped us with parallel eigensolvers. Professors W.D. Allen and H.F. Schaefer III worked with us on spectroscopic applications. We work with Dr. D.J. Dean on developing CC codes for nuclear structure. In collaboration with Dr. S. Hirata and Professor M.S. Gordon, we have developed a plan to use Tensor Contraction Engine in automated implementation of our new CC methods in the GAMESS and NWChem packages.

For further information on this subject contact:
Dr. P. Piecuch
Department of Chemistry
Michigan State University
East Lansing, MI 48824
Phone: 517-355-9715, ext. 223
E-mail: piecuch@cem.msu.edu
Internet: www.cem.msu.edu/~piecuch/group_web

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