Chemistry Reference
In-Depth Information
Alternatively,. and. despite. long-recognized. deiciencies,. extended. Huckel. (EH).
theory. is. still. frequently. employed. in. electronic. structure. calculations. of. periodic.
and. non-periodic. systems,. mainly. due. to. its. simplicity. and. the. chemical. insight. it.
provides.[12,13]..In.most.classical.EH.studies,.the.parameters.of.the.model.are.cre-
ated.with.the.(rather.ambiguous).requirement.of.providing.a.reasonable.description.
of.the.electronic.structure.of.the.system..In.recent.years.a.number.of.different.EH.
methodologies.based.on.DFT.have.been.introduced.and.successfully.applied.[14,15].
In.our.own.work,.we.have.taken.some.of.the.lessons.learned.in.the.development.
of.DFT.TB.methods.and.developed.a.new.EH.scheme.[16,17]..Contrary.to.popular.
belief,.we.ind.that.the.EH.methodology.can.provide.a.similar.level.of.accuracy.to.
competing. methods. in. many. problems,. while. preserving. many. important. advan-
tages.inherent.to.the.formalism,.namely:.(i).a.considerable.reduction.in.the.number.
of. itting. parameters,. (ii). natural. scaling. laws. for. atomic. orbitals,. and. (iii). a. rea-
sonable.transferability.of.parameters.that.allows.for.their.use.in.different.chemical.
environments.
The.most.tedious.part.of.the.applications.of.any.TB.method.is.its.parameterization,.
and.even.today,.more.than.50.years.after.the.seminal.work.of.Slater.and.Koster,.it.
remains.the.greatest.challenge.to.the.application.of.TB.theory..No.one.has.developed.
a.simple.method.to.solve.this.critical.problem..The.conventional.approach.typically.
proceeds. by. identifying. a. series. of. reference. data. that. should. be. reproduced. by. a.
parameter.set..Such.data.may.come.from.experiments,.from.other.theoretical.calcula-
tions.(typically.using.a.more.accurate.theoretical.model),.or.from.a.combination.of.the.
two..The.model.must.then.be.it.to.these.data.using.some.sort.of.optimization.tech-
nique,.often.at.a.considerable.cost.in.human.and.computer.time..Reducing.the.amount.
of.human.intervention.is.a.highly.desirable.future.goal..In.all.of.the.current.method-
ologies,. the. parameterization. process. involves. a. trial-and-error. approach. requiring.
many.iterations.before.suitable.parameter.sets.are.obtained..Although.we.do.not.have.
a.deinitive.solution.to.this.problem,.we.present.some.ideas.on.the.automation.of.this.
very.consuming.time.process.in.this.manuscript,.gleaned.from.the.lessons.learned.in.
the.course.of.our.works.in.the.last.few.years..Nevertheless,.we.acknowledge.that.much.
work.in.this.area.remains.before.a.suitable.method.will.be.found.
To.illustrate.the.use.of.TB.methods,.we.compare.the.application.of.different.den-
sity.functional-based.TB.methods.to.the.problem.of.unbiased.global.optimization.of.
neutral.atomic.gold.clusters.[18-20]..Experiments.provide.only.indirect.information.
on. the. structure. of. metallic. clusters.. TB. methods. are. particularly. suitable. for. the.
study.of.metal.clusters.in.part.because.the.number.of.isomers.grows.rapidly.with.the.
size.of.the.cluster.and.the.task.of.identifying.the.lowest.energy.structure.is.a.chal-
lenging.computational.problem..Interest.in.gold.clusters.derives.from.their.unusual.
catalytic.properties,.in.contrast.with.that.of.the.bulk.metal,.and.their.selective.bind-
ing.to.many.biomolecules,.including.DNA..Also,.gold.clusters.have.many.applica-
tions.in.nanoelectronics..Despite.the.signiicant.effort.that.has.been.invested.in.this.
subject,.there.are.still.many.open.questions..One.interesting.question.is.whether.the.
size.of.gold.clusters.changes.upon.transition.from.a.planar.structure.to.a.3D.struc-
ture..In.connection.with.this.problem,.recently.we.introduced.a.non-self-consistent.
energy.correction.that.is.based.on.the.luctuation.of.the.density.that.can.considerably.
improve.the.description.of.small.clusters.[17].
Search WWH ::
Custom Search