PNNL-13501 - Pacific Northwest National Laboratory

Mixed Hamiltonian Methods for Geochemical Electronic Structure Studies
Study Control Number: PN00066/1473
Eric J. Bylaska, James R. Rustad, Michel Dupuis
Oxidation/reduction (redox) processes at mineral surfaces are some of the most important and least understood processes
affecting the fate and transport of contaminants in the subsurface. Redox processes are difficult to understand because
electron exchange between oxidants and reductants can involve multiple pathways and numerous intermediate species
and structures, all of which can vary with geochemical conditions. Computational methods are being developed in this
project that will allow us to study important redox processes at solvated mineral surfaces.
Project Description
The purpose of this project is to develop a combined
plane-wave quantum mechanics/molecular mechanics
methods to study important geochemical processes
(structures and reactivity) at solvated redox-active natural
mineral surfaces and interfaces. When developed, this
capability will allow us to simulate geochemically
important interfacial processes that are not easily
amenable to classical models. These include changes in
oxidation states, changes in coordination and oxidation
states (bond breaking and bond formation), and redox
reactions of pollutants such as chlorinated hydrocarbons
on iron-oxides and other environmentally important
surfaces. To do this, we are combining Blöchl’s
projector-augmented-wave formalism with the popular
mixed Hamiltonian techniques of quantum chemistry to
create a unique capability for a quantum treatment of
increasingly complex and realistic models of interfacial
processes. During the first phase of the project, we have
developed a massively parallel code employing Blöchl’s
projector-augmented-wave method, and have begun
incorporating a classical treatment of solvating molecules
into it. In addition, the project has developed a method
for implementing free-space boundary conditions into
plane-wave methods, performed calculations of ironoxides
using traditional pseudopotential plane-wave
methods (not projector-augmented-wave).
Introduction
Modeling redox-active mineral/water interfaces is
difficult for several reasons. First, redox active mineral
surfaces are difficult to treat at a first-principles level, and
the addition of solvent and bulk mineral makes it even
more arduous. Accurate simulations of the reduction of a
pollutant, such as carbon tetrachloride, at the mineral/
water interface will require an accurate quantum
138 FY 2000 Laboratory Directed Research and Development Annual Report
mechanical description of the water-pollutant-surfacecomplex.
Moreover, the simulation region must be
exceedingly large so that the reaction involves the
chemical participation of solid and water.
In recent years, there have been a number of significant
advances that facilitate quantum mechanical modeling of
reduction processes at these interfaces. One recently
emerged technique, Blöchl’s projector-augmented-wave
method, is an all-electron approach that has many of the
advantages of typical pseudopotential plane-wave
approaches with the added ability to handle first-row
elements with d electrons (Blöchl 1994). Another
emerging technique, the quantum mechanics/molecular
mechanics approach, combines first-principles
simulations with classical modeling, to include long-range
effects of solvent and bulk (Gao and Furlani 1995). The
computational cost of such simulations is thus greatly
reduced by representing the solvent molecules classically.
In the quantum mechanics/molecular mechanics
approach, the system is broken up into two parts: a
localized quantum mechanical region surrounded by a
molecular mechanical region (Figure 1). This will allow
for a water-pollutant-surface-complex to be modeled
quantum mechanically, while at the same time the longrange
effects of solvent and bulk mineral can be included
with classical modeling.
Results and Accomplishments
Progress was made in several tasks on this project. A
parallel projector-augmented-wave code was developed, a
method for implementing free-space boundary conditions
into a plane-wave method was developed, a calculation of
iron-oxides using traditional pseudopotential plane-wave
methods was performed, and water pseudopotentials were
tested.