Chemistry Reference
In-Depth Information
frequently. can. be. truncated. using. terminators. to. tie. off. the. dangling. bonds. at. the.
edge. of. a. cluster. model,. such. as. modiied. hydrogen. atoms. (“siligens”). for. silicon..
For.many.years.in.surface.science,.calculations.of.adsorption.on.metal.surfaces.were.
performed.with.quantum.chemistry.codes.that.simulated.molecular.adsorption.on.a.
cluster,.using.a.molecule.bonded.to.a.small.number.(as.few.as.one!).of.metal.atoms.to.
model.adsorption.to.a.metal.surface..Metals,.however,.possess.delocalized.electrons;.
thus,. it. is. dificult. to. truncate. a. cluster. model. of. a. surface. that. does. not. introduce.
quantitatively.intolerable.boundary.effects..Indeed,.it.can.be.exceedingly.dificult,.if.
not.impossible,.to.assess.the.magnitude.of.edge.effects.on.the.computed.chemistry.
for.a.given.cluster.model.for.metals..This.assessment.must.be.done.on.a.case-by-case.
basis.for.every.cluster.model.
In.recent.years,.advances.in.density-functional-based.computational.methods.and.
improved. computational. capabilities,. particularly. the. emergence. of. massively. par-
allel. computing,. have. enabled. relatively. routine. DFT. calculations. of. bulk. systems.
consisting.of.several.hundreds.of.atoms..With.this.scale.of.calculation,.it.becomes.
practical.to.represent.chemistry.at.surfaces.within.a.super-cell.approximation.
The.local.process.of.interest.at.a.surface.(such.as.adsorption.of.a.carbon.monoxide.
molecule.on.a.platinum.surface).is.represented.as.periodically.replicated.copies.of.
the.chemical.system.on.a.metal.slab..With.a.larger.number.of.atoms.in.the.super-cell.
model,.a.molecule.at.the.surface.can.be.made.progressively.better.isolated.from.its.
periodic.copies..This.is.important.for.using.super-cell.calculations.to.model.isolated.
adsorbates.or.defects.at.surfaces.
It. is. the. purpose. of. this. section. to. describe. issues. that. need. to. be. confronted.
in.order.to.translate.this.conceptual.Ansatz.into.a.useful.computational.model.for.
quantitatively. describing. surface. chemistry.. Modeling. chemistry. of. a. molecule. on.
a. metal. surface. using. periodic. boundary. conditions. in. a. super-cell. approximation.
entails.a.number.of.complications.and.approximations.that.do.not.arise.in.quantum.
chemistry. calculations. for. inite. molecular. systems.. These. complications. unavoid-
ably.include.needing.to.understand.how.to.perform.suficiently.accurate.calculations.
of.bulk.metal.properties..But.the.true.complicating.factor.is.adapting.these.compu-
tational.tools,.fundamentally.designed.for.periodic.bulk.crystalline.systems,.to.treat.
problems.that.lack.that.periodicity..In.the.following,.we.will.go.through.a.discussion.
of.a.variety.of.issues.that.must.be.considered.when.constructing.meaningful.calcula-
tions.of.metal.surface.properties.and.chemistry.at.metals.surfaces.
6.4.1 S
laB
m
odel
of
a
S
urface
As. a. practical. matter. in. any. three-dimensional. (3D). periodic. code,. a. computa-
tional. model. of. a. surface. is. constructed. as. a. 3D. set. of. periodically. repeated. two-
dimensional.(2D).slabs.of.bulk.metal,.as.illustrated.in.Figure.6.3..We.will.describe.
how.the.positions.and.types.of.atoms.are.chosen.in.modeling.a.surface.of.choice.
The.model.of.a.surface.as.replicated.slabs.has.several.immediate.important.con-
sequences.for.the.simulation..First,.there.is.not.one.surface.in.the.slab.model,.but.
two,.one.on.the.top.of.the.slab.and.a.second.on.the.bottom.of.the.slab.above.the.cell..
This.complicates.any.analysis.as.a.second.surface.is.inextricably.entwined.into.the.
computational.model,.and.it.can.be.dificult.to.determine.the.properties.of.a.given.
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