physorg: representation/simplification of many-electron calculations through two electrons
 Breaking old barrier to better electron representation in molecular
 November 17, 2004
 Results apply to fields ranging from medicine to superconductivity
 University of Chicago quantum chemist David Mazziotti has proposed a new
 research tool that could help scientists more rapidly solve problems in
 atmospheric chemistry, combustion, medicine and other areas of research
 where the behavior of electrons plays a key role.
 "We're in the pioneering stage so we're not going to go and treat all of
 these problems right away," said Mazziotti, an Assistant Professor in
 Chemistry. But with his new method, "we can do chemistry that cannot be
 done otherwise," he said.
 Mazziotti explains his method in the Nov. 19 issue of Physical Review
 Letters. Further details will follow in early December in the Journal of
 Chemical Physics.
 The key to understanding whether or not a particular chemical reaction
 will occur depends on a detailed statistical description of the
 electrons' positions in the molecules involved. Until now, scientists
 have found it necessary to attempt to represent the motion of all the
 electrons in the molecule of interest-a daunting task requiring vast
 quantities of computer power. "Just a single water molecule has 10
 electrons," Mazziotti said.
 But in the 1950s, researchers theorized that it should be possible to
 accurately and more efficiently calculate the electronic properties of a
 molecule using only a pair of electrons representing many-even
 hundreds-of electrons in a molecular system. Mazziotti compares the feat
 to assembling a set of architectural blueprints, which represent in two
 dimensions a structure that can be built in three dimensions.
 An architect follows certain rules to ensure that a builder can
 translate a two-dimensional sketch into a three-dimensional structure.
 "In the same way atoms and molecules consist of many electrons, but
 there is a way to represent all of the electrons rigorously with only
 two electrons. Certain rules have to be followed to ensure the
 two-electron 'sketch' of the molecule accurately represents all the
 electrons in the atom or molecule," Mazziotti explained.
 Mazziotti's Physical Review Letters paper realizes a dream that
 scientists have pursued for 50 years by introducing a set of
 instructions for accurately and efficiently computing with a pair of
 electrons that represent the many electrons of the molecule. These
 instructions dramatically reduce the amount of computer time and memory
 required to compute the electronic properties of a molecule. Now
 Mazziotti can do some of the same calculations on his desktop computer
 that previously required Japan's Earth Simulator, the world's largest
 "David has really made a huge contribution in turning the dreams of 50
 years ago into useful tools," said Bob Erdahl, a professor of
 mathematics at Queens University in Kingston, Ontario. Erdahl said
 Mazziotti's Physical Review Letters paper has applications to his own
 research in computing how behavior at the subatomic level brings about
 macroscopic changes in materials, such as the transition to the
 superconducting state.
 "I'm certainly going to look very closely and try to incorporate David's
 latest innovation into my work. I think we will very quickly be able to
 beat other approaches in this area of solids and compute things that
 were out of reach before," Erdahl said.
 Erdahl is especially interested in determining why superconductivity
 manifests itself only in two-dimensional layers rather than in
 three-dimensional solids. "The computations are of course very difficult
 to do. These methods that David is developing and that we're developing
 are very helpful in attacking that problem."
 While Erdahl works in mathematical physics, understanding the electronic
 energies that atoms and molecules possess also affects almost every area
 of chemistry. One such area, Mazziotti said, has broad applications
 includes the chemistry of free radicals-highly reactive unpaired
 In atmospheric chemistry, free radicals are instrumental in reactions
 leading to ozone depletion and the creation of greenhouse gases. Another
 area is the combustion of hydrocarbon fuels, which creates a variety of
 carbon-based radicals.
 "A lot of people want to know which radicals are present in a given
 combustion process and what reactions those were undergoing because
 that's going to affect fuel efficiency," Mazziotti said.
 A third area is medicine, because radical-type reactions are common in
 the human body. Mazziotti noted that hydroxy urea therapy combats
 sickle-cell anemia by forming a radical that triggers a cascade of
 additional reactions. "There is a 40 percent reduction in mortality for
 patients who receive hydroxy urea treatment for sickle cell," he said.
 Despite the advances that Mazziotti and others have contributed to the
 representation of electrons in atoms and molecules, further advances
 could be in the offing. He said the field is experiencing a new wave of
 research. "We're not done by any means."
 Source: University of Chicago
 Eugen* Leitl
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