Computational physics

The Physics Department at Wake Forest University has a strong program in computational and theoretical physics. The computational and theoretical efforts span from computational biophysics to theoretical gravitational physics and computational condensed matter research. The following faculty is involved in computational and/or theoretical physics research:

Paul Anderson and his students apply quantum field theory to the study of black holes and cosmology. The presence of a strong gravitational field results in some interesting and unusual phenomena. These include the acquisition of a nonzero energy by the vacuum and the creation of particles. In the case of black holes, such effects can both distort the spacetime geometry near a black hole and cause it to slowly lose its mass. This group is computing the distortion of spacetime near black holes. The dynamical evolution of the early universe may have been strongly affected by the existence of nonzero vacuum energy and particle production. Anderson is currently interested in the details of how particle production reheats the universe after an inflationary phase. He is also interested in quantum effects that occur during periods such as inflation when the universe is accelerating.

Eric Carlson's research covers a variety of topics including both particle phenomenology and astrophysics. For the next decade or so, astrophysics will dominate discoveries in particle physics, since there are very few opportunities at high energy colliders for new discoveries.

Two of his recent papers have focused on the conversion of pseudoscalars into photons in the presence of large scale or strong magnetic fields. Such conversions may provide the best limits on pseudoscalar couplings. Up to now, he has focused on methods that rely on the galactic magnetic field, and consequently are sensitive only to massless pseudoscalars. He hopes to use some of the same ideas in the context of stellar magnetic fields, allowing the probing of much larger masses. He hopes to improve limits on the axion by this technique.

One area he has focused on recently is the possibility of a naturally small cosmological constant. A positive cosmological constant could help resolve the apparent discrepancy between the large estimated ages of stars in globular clusters and the rather large value of the Hubble constant coming from most recent experiments. It is difficult to see how such a small cosmological constant could arise. In a paper with W.D. Garretson, he explored the possibility that the universe might have a zero cosmological constant in the true vacuum, but that we might lie in a false vacuum. The smallness of the splitting between the two vacua could arise due to the appearance of an accidental discrete symmetry connecting the two vacua. The splitting would be the result of Planck-scale suppressed nonrenormalizable terms which would violate the discrete symmetry. Carlson hopes to explore models which demonstrate these ideas in a more natural way in the near future.

Neutrino physics is another area that interests Carlson. He has written several papers involving various neutrino mass generation mechanisms, and he is returning to this subject once more. Neutrino masses naturally arise in many extensions of the standard model, and could have important cosmological consequences. With new results from various solar neutrino and terrestrial neutrino experiments, and the appearance of new detectors in the near future, this will be a rapidly growing field.

Carlson believes astroparticle physics to be one of the most promising areas in particle phenomenology in the next decade or two.

Greg Cook's research interests are in the areas of computational astrophysics and gravitational physics. Currently, his research is centered on studying the coalescence of compact binary systems. Compact binaries are systems containing a pair of black holes or neutron stars that orbit each other. Eventually, the objects in these systems must collide since they are constantly loosing energy in the form of gravitational waves. These gravitational waves that are produced as these compact objects spiral closer together, as well as those that are produced when they collide and settle down, will be observed in the latest generation of laser-interferometer gravity wave detectors that are currently under construction. By observing these collisions, it is expected that we will learn a great deal about the physics of strong gravitational fields, the behavior of ultra-dense matter, the expansion of the universe, and much more.

Cook and his collaborators are currently developing the theoretical and computational tools needed to simulate the collision of a pair of black holes. Numerical simulations are the only way to study the last few orbits and the ultimate coalescence of a compact binary system, and so, they are the only way to connect theory with experimental results. Some of the issues that are currently being studied are: the computation of initial conditions for black hole and neutron star binary systems, the formulation of stable evolution schemes, and locating the surfaces of black holes as they evolve. All of these problems are heavily computational and require the use of supercomputers such as the Physics Department's IBM SP2.

The primary interest in Jacquelyn Fetrow's laboratory is understanding the relationship between protein structure, motion and function. A better understanding of the inter-relationship between these features will be valuable in the design of better and more specific compounds for the pharmaceutical drug discovery process. Current research projects focus on using computational tools for studying protein structure and dynamics. Two specific projects are ongoing: a survey of protein structures in functional families and molecular dynamics simulations of pharmaceutically-relevant protein families. In the first project, the goal is to identify areas of structural plasticity in proteins of similar or related function and to correlate those regions of plasticity with the location of functional sites and the chemistry of those functional sites. In the second project, the goal is to model the actual motion of protein structures, to correlate that motion with functional site chemistry, and then to better understand how those motions change when an inhibitor or drug binds to the protein. For more information, see Prof. Fetrow's research page.

Natalie Holzwarth and Rick Matthews lead research groups that conduct first principles computer modeling of electronic and structural properties of materials. They work closely with experimental groups on a variety of projects including the study of new materials, surfaces, and defects in crystals.

Most of the modeling is based on density functional theory, developed by Walter Kohn and his collaborators in 1964 (for which he shared the 1998 Nobel prize in Chemistry). The work is divided between a focus on code and algorithm development and a focus on materials physics.

The group has developed some general purpose electronic structure code which is made available on the web at http://pwpaw.wfu.edu.

Some of the recent materials studied by the group include scintillating crystalssuch as PbWO₄, F-centers in alkali halide crystals, and surface states related to photoemission experiments performed in Richard Williams' lab. Other projects include studies of molecular crystals and Li ion battery materials such as the cathode materials (FeLiPO₄, FePO₄) and electrolytes (Li₃PO₄).

William Kerr leads a group in the study of statistical physics. The research in this group concentrates on systems in which nonlinearity is the dominating feature. In some situations nonlinearity produces highly organized behavior, which is described by solitons. In other cases nonlinearity produces seemingly random behavior, which is often characterized as chaos. This group studies these phenomena by computer simulations. Their emphasis recently has been on studying soliton mechanisms underlying the dynamics of first- and second-order phase transitions. The experimental impetus for these studies comes from structural phase transitions in solids. The major issues here include the existence or nonexistence of soft modes and of precursor fluctuations of the product phase occurring within the parent phase. Such fluctuations would be described by nonlinear partial differential equations similar to the sine-Gordon or nonlinear Schrodinger equations, perhaps with extensions which describe bond anharmonicity in addition to the site anharmonicity. In addition to these important physical questions, employing computer simulation to study these problems raises a set of methodological problems. These deal with writing computer codes that efficiently use vector supercomputer architectures and with developing methods of analysis, including visualization schemes, that effectively extract the important physical processes occurring in the system from the large volume of numbers produced by the simulations .

Fred Salsbury's research focuses on theoretical biophysics and chemical physics. In particular, current interests are in understanding the relationships of biomolecular dynamics and conformational flexibility to biological function. . Exploring the molecular basis of such relationships requires the development and application of new methods in classical mechanics, quantum mechanics and statistical mechanics. Recent methodological work has included the development of Magnetic Field Density Functional Theory, improvements in Generalized Born continuum solvation theory, and creation of a new implicitmodel for the study of pH and charge effects. Recent applications have included studying the origin of the magnetic-field-induced-quadruple shift, studying the ligand-inducedconformational and dynamical changes and quantum chemistry of some penicillin degrading enzymes, and exploring the unusual photochemistry found in some antigen-antibody complexes. Future research directions will involve the examination of two interrelated areas. One is further exploration of the links between dynamics, conformational change, and catalysis in proteins. The second, improving our understanding of electrostatic effects on protein stability and dynamics, e.g. pH effects and unraveling the roles of explicit hydration versus continuum solvation, with an aim towards eventual studies of the interactions and assembly of proteins and other biomolecules.

The group of Timo Thonhauser conducts research in theoretical and computational condensed-matter physics with a focus on the development of ab-initio electronic-structure methods and their application to bio-, nano-, and energy-related materials. These theoretical studies go hand-in-hand with experimental research and provide the necessary framework to understand the behavior and characteristics of materials. Such knowledge is the basis for the design of new, improved, and advanced materials with direct applications to all areas of technology.

The research in Thonhauser’s group usually has three components. The first component is model development, which in many cases is based on the theory of quantum mechanics. The next step is the translation of this theoretical model into a computer program, which involves the development of algorithms as well as the development of computer codes appropriate for parallel computing on super computers. The last step then is the application of theory and code to problems of current interest.

One project of current interest centers on the development of a new method for calculating ab-initio NMR chemical shifts, with a special emphasis on large systems such as bulk water, intercalated DNA, and proteins. Another project focuses on the calculation of the orbital magnetization in periodic crystals, in particular, simple metals like nickel, iron, and cobalt. Yet another project of current interest aims to extend density functional theory to include van der Waals interactions—which play an important part in many organic, biological, and nano materials—with applications ranging from hydrogen storage to DNA.