Chemistry Reference
In-Depth Information
6.1 INTRODUCTION
The. aim. of. this. chapter. is. to. introduce. the. basic. challenges. and. tools. involved.
in.modeling.chemistry.on.metallic.surfaces,.and.to.provide.practical.guidance.in.
the. use. of. these. methods. and. tools. to. design. quantitative. simulations. of. surface.
processes..Calculations.involving.a.metal.surface.introduce.many.additional.com-
plications.relative.to.quantum.chemistry.studies.of.molecular.chemistry..These.not.
only. include. new. methods. and. unfamiliar. tools. but. also. novel. issues. and. unex-
pected. pitfalls.. Thanks. to. continuing. advances. in. methods. and. computational.
capabilities,.irst.principles.simulations.of.surface.chemistry.are.becoming.gener-
ally.accessible..These.advances.are.founded.upon.the.success.of.density.functional.
theory.(DFT).and.the.development.of.large-scale.solid-state.codes.implementing.
DFT.for.periodic.systems..The.size.and.complexity.of.irst.principles.simulations.
has.become.staggeringly.large.compared.to.what.was.feasible.decades.ago,.yet.the.
codes.can.be.deceptively.simple.to.use..The.advanced.capabilities.of.modern.com-
putational.tools.rely.on.many.layers.of.complexity.with.many.hidden.options.and.
assumptions,.and.it.is.possible.that.concealed.approximations.may.ultimately.limit.
the.accuracy.of.a.simulation..Obtaining.quantitatively.meaningful.calculations.for.
surface. chemistry. requires. awareness. of. the. physical. approximations. inherent. in.
the. methods. and. careful. design. of. a. viable. model.. Improper. use. of. these. tools.
can.result.in.drastically.unrealistic.conclusions..Some.general.considerations.along.
these. lines. for. solid-state. calculations. in. materials. science. were. considered. in. a.
recent. review. article. [1].. Here,. we. describe. several. fundamental. issues. that. arise.
in.applying.DFT.methods.to.surface.chemistry,.calculations.that.explore.the.inter-
section.between.physics.and.chemistry,.the.realm.of.metallic.clusters.and.surface.
catalysis.
This.chapter.is.divided.into.three.sections..The.irst.details.important.concep-
tual.differences.between.inite.molecular.systems.and.bulk.metals,.and.the.impli-
cations.these.differences.have.on.the.computational.description.of.the.chemistry.at.
metal.surfaces..The.second.gives.a.brief.overview.of.DFT,.focusing.on.the.impor-
tance. of. the. choice. of. functional. can. have. on. results,. and. how. the. proper. inter-
pretation.of.DFT.may.limit.the.scope.of.conclusions.that.can.be.drawn.from.the.
results..The.third.section.describes.the.design.of.a.viable.computational.model.for.
surface. chemistry:. the. use. of. a. slab. model. to. describe. the. surface,. construction.
of. an. appropriate. simulation. cell,. testing. and. verifying. the. evaluation. of. surface.
properties,. and. use. of. the. model. to. study. chemistry.. This. process. is. illustrated.
through.detailed.examination.of.two.surface.chemistry.examples:.erbium.surfaces.
and.CO.on.Pt.
6.2 CHALLENGES ARISING IN THE SIMULATION OF METALS
The.deining.difference.between.metals.and.nonmetals.is.the.lack.of.an.energy.gap.
between.the.ground.and.excited.states..In.the.molecular.orbital.picture,.this.transi-
tion.from.nonmetals.to.metals.can.be.thought.of.as.a.vanishing.energy.difference.
between.the.highest.occupied.and.lowest.unoccupied.molecular.orbital,.the.HOMO-
LUMO.gap..Once.this.gap.is.closed,.there.is.no.energy.barrier.to.prevent.electrons.
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