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     COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
AND THE SCHOOL OF PHYSICS, ASTRONOMY, & COMPUTATIONAL SCIENCES
                            (CSI 898-Sec 001)
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     Coupling quantum mechanics and interatomic potentials in
                    	    solids and liquids

	                        Noam Bernstein
  Center for Computational Materials Science, Naval Research Laboratory
                         Washington, DC

Accurately describing material properties during processes that involve bond
breaking, for example fracture or a chemical reaction, requires an atomistic
description of the material, and a quantum-mechanical description of the
electrons that mediate bonding.  Density functional theory has made it
feasible to carry out such simulations reliably from first principles, but is
limited in practice to a few hundred atoms.  However, in many interesting
systems the localized bond breaking process is coupled to a very large number
of atoms.  In fracture this coupling is mechanical, to the applied strain
field, while for reactions in water the bond breaking is coupled
electrostatically to the transient hydrogen bond network of the solvent
molecules.  I discuss the issues that occur in trying to treat such coupled
systems, and describe the buffered force-mixing method we have developed to
overcome these problems.  I present results on the first application of the
method, to fracture in silicon, a model brittle solid, where it predicts a low
speed instability that can be linked to experimentally observed features of
the crack surface.  I also show how the method has been extended to describe
reactions in liquids, where it allows solvent molecules to affect the reaction
and diffuse into and out of the quantum-mechanical region.  I present results
for the structure of liquid water and a proton transfer reaction in water that
shows good agreement with fully quantum-mechanical calculations, in contrast
with conventional coupling methods.

                          October 1, 2012
                               4:30 pm
                    Room 301, Research I, Fairfax Campus

Refreshments will be served at 4:15 PM.
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