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SettingupSCFparametersSCFsettingsdeterminethealgorithmthatCASTEPusestofindthegroundstateoftheelectronicsubsystem,aswellastheaccuracyrequired.Mostofthesesettingsdonotneedtobechangedbytheuser.Forexample,theSCFtoleranceiscontrolledbytheglobalQualityoptionontheSetuptab.ItcanalsobemodifiedusingtheSCFtoleranceoptionontheElectronictab,howeverthisisnotrecommended.Similarly,themaximumnumberofSCFcycles,whichcanbeadjustedusingMax.SCFcyclesoptionontheSCFtabontheElectronicOptionsdialog,neednotbechangedundernormalcircumstances.TheMax.SCFcyclessettingdetermineshowmanySCFstepsaretakenbyCASTEPbeforeitmovesatomsaccordingtothetaskbeingperformed(i.e.GeometryOptimizationorDynamics).Note.TheCASTEPserverautomaticallyincreasesthenumberspecifiedintheinterfacebyafactorofthreeformetallicsystems.ElectronicminimizationalgorithmThealgorithmthatisusedtosolvetheDFTequationsisspecifiedbytheElectronicminimizeroptionontheSCFtabontheElectronicOptionsdialog.Densitymixingistherecommendedchoice,intermsofbothrobustnessandefficiency.Wefoundittobe2-4timesfasterforinsulatorsthantheconjugate-gradientbasedAllBands/EDFTscheme.TheDensitymixingschemeisespeciallygoodformetallicsystems,wherespeedupsformetalsurfacescomparedtoconjugategradientschemesareintheregionof10-20.Theonlycasewheredensitymixingmaynotimproveperformanceisformoleculeinaboxcalculations.ThedefaultdensitymixingsettingsusePulaymixingandconjugate-gradientminimizationofeachelectronicstate.YoushouldonlyattempttochangetheseparametersifSCFconvergenceisverypoor.SometimesithelpstoreducethelengthoftheDIIShistoryfromthedefaultvalueof20toasmallervalue(5-7).Itmightalsobehelpfultodecreasethemixingamplitudefromthedefaultvalueof0.5to0.1-0.2.VariableelectronicstatesoccupanciesBydefaultCASTEPusesvariableelectronicoccupancies,thuseffectivelytreatingallsystemsasmetallic.Thisisrecommended,asitspeedsupdensitymixingoptimization,evenforsystemswithlargebandgaps.ThenumberofemptybandsshouldbesufficientlylargetocaterfornearlydegeneratebandsclosetotheFermilevel.Thisisrelevantfortransitionorrareearthmetalcompounds,wherenarrowdorfbandscanbepinnedattheFermilevel.Ifthenumberofbandsusedforsuchasystemisinsufficient,theSCFconvergencewillbeveryslowandprobablyoscillatory.Occupationnumbersofthehighestelectronicstatesasreportedinthe.castepfilearelikelytobenoticeablynonzeroforatleastsomek-points.SCFconvergencewiththeDensitymixingminimizationschemecansometimesbepoorformetallicsystems.Ifthisisthecase,thealternativeAllBands/EDFTscheme,whichisbasedontheensembledensity-functionaltheory(Marzarietal.,1997)offersamorerobustalternative.Tip.SlowSCFconvergenceisoftenindicativeofaninsufficientnumberofemptybands,especiallyinspin-polarizedcalculations.Tocheckifthisisthecause,inspecttheoccupanciesofthehighestenergyelectronicstates.Theyshouldbeveryclosetozeroforallk-pointsinacalculationwhichissetupcorrectly.FurtherinformationSettingupelectronicoptionsElectronic-CASTEPCalculationSCF-ElectronicOptionsDensityMixingOptions-SCF