THEORETICAL ATOMIC AND MOLECULAR PHYSICS

The research in the group is focused on the development and application of quantum theoretical techniques for the study of atomic and molecular systems. Projects range from the study of the structure and properties of molecular clusters to the development of new theoretical and computational techniques as well as more fundamental questions regarding the interpretation of quantum theory.

Molecular Clusters
Pure and mixed molecular clusters of carbon, silicon, and germanium atoms provide an interesting and challenging group of molecular systems to a theorist. Sufficiently high-level theoretical methods have to be employed to provide accurate data to be used in conjunction with the analysis of experimental data.

In these theoretical studies we employ state-of-the-art computational techniques to solve the quantum mechanical many-body electron problem in the Born-Oppenheimer approximation. These techniques includes the so called coupled cluster methods where the electronic wave-function is essentially expressed in an infinite sum with certain constraints that lead to a finite computational scheme. Relatively recently, an alternative approach, the density functional method (DFT), has been developed for the description of electronic ground states. Here, instead of attempting to describe the electronic wave function, the focus is on calculating the electron density. Such DFT techniques can provide very accurate information at a very modest computational cost and enable us to study and describe large molecular clusters more accurately.

Although advanced software is available for electronic structure calculations an additional challenge is to provide results to experimentalists that are meaningful in that they come with some type of “error bars” to facilitate in the comparison with real experimental results. One of the major goals in the group is to develop and employ techniques in a way to facilitate the resolution of experimental spectra. As a result a number of new theoretical techniques that serve as interfaces between theory and experiment have been developed.

Fourier Transform Infrared Observation of the Vibrational Spectrum of the Linear SiCCH Radical in Ar at 10 K (with D.S. Han and W.R.M. Graham), J. Chem. Phys. 106, 6222 (1997).

Detection of Cyclic Carbon Clusters I: Isotopic Study of the v₄(e_u) Mode of Cyclic C6 in Solid Ar (with S.L. Wang and W.R.M. Graham), J. Chem. Phys. 107, 632 (1997).

Detection of Cyclic Carbon Clusters II: Isotopic study of the v₁₂(e_u) Mode of Cyclic C8 in Solid Ar, (with S.L. Wang and W.R.M. Graham), J. Chem. Phys. 107, 725 (1997).

Electronic Structure Methods
Quantum theoretical and computational methods are developed and refined in order to perform efficient calculations of the electronic structure of atoms and molecules. Our main interest has been in coupled cluster methods and the closely related many-body perturbation theoretical techniques.

Photoelectron Spectroscopic and Theoretical Study of Ketene Imine CH₂=C=NH and Ketene N-Methylimine CH₂=C=NCH₃, (with H.W. Kroto, G.Y. Matti, R.J. Suffolk, J. Watts, and R.J. Bartlett), J. Am. Chem. Soc. 112, 3779 (1990).

Fock Space Multireference Coupled-Cluster Theory for General Single Determinant Reference Functions, (with J.F. Stanton and R.J. Bartlett), J. Chem. Phys. 97, 5560 (1992).

Fundamental Quantum Theory
Quantum theory is a well-established theory which provides a highly accurate description of microcosmic phenomena. Although developed in the early part of the 20th century several problems concerning the interpretation of quantum theory still remain. Ongoing projects involve the study of the structure of the theory in the complex energy plane using complex scaling techniques as well as investigations of new and alternative descriptions of the quantum theory measurement.

Generalized Green's Functions and Spectral Densities in the Complex Energy Plane, (with E. Engdahl, E. Brändas, and N. Elander), J. Math. Phys. 27, 2629 (1986).

Resonances and Background. A Decomposition of Scattering Information, (with E. Engdahl, E. Brändas, and N. Elander), Phys. Rev. A 37, 3777 (1988).