Chemistry Reference
In-Depth Information
We.have.seen.that.the.choice.of.functional.is.wrought.with.uncertainty.and,.typi-
cally,.greater.accuracy.requires.a.greater.computational.cost.and.can.result.in.a.less.
reliable.calculation..A.general.review.of.the.quantitative.performance.of.many.func-
tions.was.published.recently.[41]..Now,.as.we.embark.on.the.perilous.application.of.
DFT.to.a.practical.calculation,.the.choice.of.functional.is.only.one.in.a.myriad.of.
decisions.a.researcher.must.make.
6.4  SIMULATING CHEMISTRY AT METAL SURFACES
Simulations.of.chemical.processes.occurring.at.metal.surfaces.lie.at.the.crossroads.
of. quantum. chemistry. and. solid-state. physics,. presenting. the. challenges. of. two.
domains..On.one.hand,.one.is.interested.in.the.chemical.behavior.of.a.molecular.sys-
tem.with.short-ranged.electronic.interactions,.the.focus.of.computational.chemistry..
On.the.other.hand,.behavior.below.a.surface.is.dictated.by.the.bulk.properties.of.the.
substrate..For.the.highly.mobile.electrons.of.a.metal,.the.focus.of.solid-state.physics,.
the.range.of.interaction.can.be.rather.long..While.nominally.concerned.with.elec-
tronic.structure.through.the.solution.of.the.Schrödinger.equation,.the.two.branches.
are.built.upon.rather.independent.theoretical.frameworks.and.mostly.distinct.sets.of.
computational. tool. and. techniques. owing. to. the. different. approximations. that. are.
reasonable.to.make.in.each.discipline.
There. exists. a. vast. computational. infrastructure. developed. over. the. past. sev-
eral. decades. that. enables. highly. accurate. calculations. of. molecular. properties..
Semiempirical.methods,.such.as.Hückel.theory.[2],.tight-binding.[4],.and.neglected.
overlap.methods.[42],.are.useful.for.quick.estimates.of.many.properties,.including.
excited.state.properties..More.expensive.methods,.such.as.HF,.have.been.the.main-
stay.of.quantum.chemistry.for.many.years..More.costly,.multiconigurational.meth-
ods,.such.as.coniguration.interaction.and.coupled.cluster.treatments,.enable.almost.
arbitrarily. accurate. predictions. of. molecular. electronic. structure.. Additionally,.
today's.hybrid.density.functionals,.such.as.B3LYP,.have.proven.to.be.effective.and.
eficient.computational.workhorses.for.predictions.for.molecular.chemistry.
In.recent.years,.there.has.been.an.analogous,.almost.completely.independent.com-
putational. infrastructure. developed. for. simulations. of. condensed. matter. systems,.
with.DFT.being.the.workhorse.method..The.computational.demands.of.extended.sys-
tems.limit.the.accuracy.of.the.methods.that.can.be.applied.in.tractable.simulations.
The. challenge. of. modeling. surface. chemistry. lies. in. successfully. negotiating.
the. compromises. of. simultaneously. satisfying. the. constraints. of. achieving. sufi-
cient.accuracy.required.for.molecular.chemistry.and.the.demands.of.incorporating.
extended. bulk. metal. effects. on. that. chemistry.. The. perfect . calculation. is. not. pos-
sible;.the.Schrödinger.equation.in.its.unapproximated.form.being.unsolvable.for.bulk.
systems..A.deliberate.judgment.must.be.made.as.to.what.compromises.will.lead.to.
acceptable.errors.in.the.simulation,.with.a.rigorous.assessment.of.the.magnitude.of.
those.errors.and.their.impact.on.the.conclusions.drawn.from.the.simulations.
Smaller. computational. models,. such. as. modeling. a. bulk. material. as. a. cluster,.
permit.more.accurate.quantum.chemistry.methods.to.be.applied.at.the.expense.of.
neglecting. bulk. effects.. With. semiconductor. or. oxide. substrates,. where. the. elec-
tronic.structure.can.be.well.approximated.as.localized.bond.pairs,.a.substrate.model.
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