Lab:GettingstartedwithORCA.InthislabwewillexploreanumberofbasiccalculationsusingtheORCAprogram.ORCAisastateoftheartquantumchemistryprogramthatisdesignedtotreatfairlylargesystems(uptoabout100-200atoms).Ithasefficientmodulestodoavarietyofdensityfunctionalcalculations(optimizations,frequencies,excitedstates),accuratesinglepointenergycalculations(DLPNO-CCSD(T),andverynicetoolstoinvestigateawidevarietyofspectroscopies(UV-vis,ESR,NMR)usingadvancedelectronicstructuremethods(CASSCF,NEVPT2,MRCI,EOMCC,STEOM-CC,MREOM-CC),includingrelativisticeffectsandspin-orbitcoupling.Averyusuefulfeatureistheuseofso-calledlocalcorrelationapproaches(DLPNO),thatallowthecalculationoflargemolecules,yieldingaccurateresults.TheNooijengrouphasbeeninvolvedinthedevelopmentofSTEOMandMREOMapproachesintheORCAprogram.Theinputfilescanbefoundonadirectoryonchem400a.Youcanbrowsethorughthisdirectoryandcopyfilesyoufindusefulstartingpoints.Thedirectroyiscalled~nooijen/Orca_examples_2017_nlogn Ithasanumberofsubdiretories.ORCAisahugeprogramwithmanyfeatures.Fordetailedinformationyoushouldconsultthemanual(seelinkonthewebsite).1.GroundstateDFTcalculations.OptimizationsandFrequencycalculations(directorygeo).Thefollowinginputfiledoesanopt+freqcalculationonformaldehyde,usingazmat(orinternalcoordinates)tospecifythegeometry.TheDFTmethodofchoiceisB3LYP,andthebasissetisdef2-SVP:! B3LYP def2-SVP Opt Freq * int 0 1 C 0 0 0 0.0 0.000 0.000 O 1 0 0 1.2 0.000 0.000 H 1 2 0 1.1 120.0 0.000 H 1 2 3 1.1 120.0 180.000 * *int01:indicatesthegeometryisspeciedininternalcoordinates(R,theta,phi).Theintegers123etcspecifytheatominquestionhasadistanceRfromatom1,makesanangletheta:(atom-1-2),andhasadihedralanglephi(atom-1-2-3).YoucanexaminetheORCAoutputfileandfindtheoptimizedgeometry(searchforHURRAY) ***********************HURRAY********************

*** THE OPTIMIZATION HAS CONVERGED *** ************************************************* Followingtheprinitngoftheconvergedgeometry,ORCAdoesafinalsinglepointenergycalculation,printingmoreinformation.SearchforthestringTOTAL SCF ENERGY. Theprogramcontinueswithacalculationofvibrationalfrequencies,whichcanbefoundunderstringVIBRATIONAL FREQUENCIES, IR SPECTRUM, THERMOCHEMISTRY. AlternativelyyoucanuseCartesiancoordinates(inAngstrom),ifyouhaveanexternalsource,e.g.! B3LYP def2-SVP Opt Freq *xyz 0 1 C 0.000000 0.000000 0.000000 O 1.199700 0.000000 0.000000 H -0.604337 0.945048 0.000000 H -0.604337 -0.945048 -0.000000 * Theabovecalculationsarethesimplestexamples,butforlargemoleculesyouwillwanttorunmoreefficientcalculations.Amoresophisticatedcalculationwouldbe! RIJCOSX RI-B2PLYP D3BJ def2-TZVP def2/J def2-TZVP/C TIGHTSCF Grid5 FINALGrid6 * int 0 1 C 0 0 0 0.0000 0.000 0.00 O 1 0 0 1.2500 0.000 0.00 H 1 2 0 1.1075 122.016 0.00 H 1 2 3 1.1075 122.016 180.00 * Thiscalculationusesa“chain-of-spheres”calculationtospeeduptheinclusionofexchange(keyword:RIJCOSX),Itusesaresolutionoftheidentityapproachwithanauxilliarybasistospeedupthecalculationofintegrals(keywords:RI-B2PLYP, def2/J def2-TZVP/C).Thecalculationsusesdifferentintegrationgridsduringoptimizationandfinalenergycalculations(keywords:TIGHTSCF, Grid5 FINALGrid6),andfinallyitusesacorrectionfordispersion(vanderWaals)interactionsdenotedbyD3BJ.YoucanreaduponallofthesekeywordsintheORCAmanual.Ifyouplantorunanumberofcalculations,itwouldpaytoexperimentto

seehowyoucanperformcalculationsinthefastestwaywithoutloosingsignificantaccuracy.2.TDDFTexcitedstatecalculations(directory:excite_mix).TheTDDFTapproachisaquiteefficientandaccurateapproachtocalculate(singly)excitedstates.ThefunctionalthatisrecommendedforexcitedstatesistheCAM-B3LYPfunctional.(Fortransitionmetalsystemsotherfunctionalsmaybemoreappropriate).Hereisasimpleinputfile(includingRIJCOSXandRI)approximations.! cam-b3lyp ma-def2-TZVP RIJCOSX def2/J Gridx5 FinalGrid6 TightSCF %maxcore 100 %tddft Ethresh 10.0 end * int 0 1 C 0 0 0 0.000000 0.000 0.000 O 1 0 0 1.199700 0.000 0.000 H 1 2 0 1.121757 122.598 0.000 H 1 2 3 1.121757 122.598 180.000 * Thema-def2-TZVPbasissetincludesdiffusebasisfunctions,whicharevitalforexcitedstatecalculations.Theprogramcalculatesallsingletstatesbelow10.0eV.Therelevantsectionoftheoutputcanbefoundundertheheading:TD-DFT/TDA EXCITED STATES (SINGLETS). Hereyoucanfindalisitngofexcitedstates,includingthetypeofexcitation,andalistofveriticalexcitationenergiesandoscillatorstrenghts.Youcanalsoobtaintripletstates(seemanual).Itispossibletodogeometryoptimizationsforaparticularexcitedstate.Imaydiscussthisinafuturelab,togetherwiththecalculationofFranck-Condonspectra.Fornow,IrefertotheORCAmanualformoredetails.3.HighlyaccurateDLPNO-CCSD(T)calculations(directory:dlpno-ccsd).InquantumchemistryweareofteninterestedinGibbsfreeenergiesofareaction.Thestandardprotocolistooptimizegeometriesandobtainvibrationalfrequenciesforallspeciesinvolvedinthereaction,usingDFTcalculationsdiscussedpreviously.NextonecandomoreaccuratesinglepointenergycalculationsandreplacetheDFTelectronicgroundstateenergieswiththemoreaccurateCCSD(T)energies,keepingthethermalenergycorrectionsfromDFT.ThemostaccurateroutinelyapplicablemethodisDLPNO-CCSD(T).Itisstillexpensivebutapplicabletolargesysytems(100-200atoms).Hereisaninputfileforourfavouritesmallmolecule.

! DLPNO-CCSD(T) cc-pVTZ cc-pVTZ/C RIJCOSX def2/J TIGHTSCF * xyz 0 1 H 0.000000 0.934473 -0.588078 H 0.000000 -0.934473 -0.588078 C 0.000000 0.000000 0.000000 O 0.000000 0.000000 1.221104 * Thefinalenergycanbefoundsearchingfor:E(CCSD(T)) 4.Accuratecalculationsofverticalexcitationenergiesforlargemolecules:DLPNO-STEOM(directory:steom_excite)TD-DFTisagoodstratingpointforexcitationenergiesandisexpectedtoyieldexcitationenergies(forwellbehavedsinglereferencemolecules)thatareaccuratetoabout0.3eV.TheSTEOMapproachinitslocalapproximation:DLPNO-STEOMisexpectedtohavesomewhathigheraccuracy(about0.1eV).Belowisaninputfile.!STEOM-DLPNO-CCSD TightSCF ma-def2-TZVP def2-TZVP/C def2/J NORMALPNO %maxcore 12000 * xyz 0 1 H 0.000000 0.934473 -0.588078 H 0.000000 -0.934473 -0.588078 C 0.000000 0.000000 0.000000 O 0.000000 0.000000 1.221104 * %mdci NRoots 8 #no of roots printlevel 3 #printing level OTHRESH 0.005 # Threshold for occupied orbital for the active space selection, different from the current defaults. VTHRESH 0.01 # Threshold for virtual orbitals for the active space selection IPSTHRS 70 # Thresh hold for discarding doubly excited IP states EASTHRS 70 # Thresh hold for discarding doubly excited EA states TCutPNOSingles 6.6e-10 # singlesPNO need to be rescaled to get good DLPNOEA DoNewActSch false # new active space selection scheme NDAV 400 # Davidsons dimension Maxiter 2000 # maxiter, probably last two is just overkilling DTol 1e-5 # Davidsons tolerence

DoROOTWISE true # use the root wise solver FollowCIS true # track CIS root end Ihaveaddedsomemorekeywords,asherethedefaultsare(were?)notaswellestablished.Youcanlookthroughtheoutputfilestofindspectralinformation.5.MultireferenceSystems(directory:NEVPT2/NArO)Thisisahugecomplicatedtopic.Inmanysystems(biradicals,transitionmetalcontainingsystems,bond-breaking)thegroundstateisnotwelldescribedwithasingleleadingdeterminant(nomattertheuseofoptimizedorbitals).Ageneralcharacteristicisthepresenceoflow-lyingexcitedstates(below1eVsay).Suchsystemsrequireamoregeneraldefinitionofareferencestate,andthedefinitionofasetofpartiallyoccupied,so-calledactiveorbitals.TheleadingkeywordsareCASSCF(theanalogueofHF),NEVPT2(theanalogueofMP2),MRCI,MREOM-CC.TheNooijengroup(studentLeeHuntington)developedandimplementedtheMREOM-CCapproachinORCA,whichisapowerfulmethodformultireferenceexcitedstates,butcurrentlyworkingmostlyforsmallmolecules.IpostedadetailedseparatelabonthisontheChem400website.HereIwillgiveasanexampleaCASSCFandNEVPT2inputfile,asastartingpoint.!CASSCF aug-cc-pvdz ExtremeSCF * xyz 0 6 N -2.18264693 0.62310170 0.00000000 AR 0.10972905 -0.60368775 0.00000000 O 2.31834787 0.76817336 0.00000000 * %casscf nel 7 norb 6 mult 6 nroots 3 end Thisinputfilecalculates3statesofspinmultiplicity6(S=2.5)inanactivespaceof6orbitals(2ponNand2ponO),with7electronsintheactivespace(3forNand4forO).ThenextcalculationdoesanaverageCASSCFonevenmorestates,andthisisfollowedbyaNEVPT2calculationofthesamesetofstates.

!CASSCF aug-cc-pvdz ExtremeSCF * xyz 0 6 N -2.18264693 0.62310170 0.00000000 AR 0.10972905 -0.60368775 0.00000000 O 2.31834787 0.76817336 0.00000000 * %casscf nel 7 norb 6 mult 6, 4, 2 nroots 3, 3, 3 end NEVPT2:!CASSCF aug-cc-pvdz ExtremeSCF NoIter * xyz 0 6 N -2.18264693 0.62310170 0.00000000 AR 0.10972905 -0.60368775 0.00000000 O 2.31834787 0.76817336 0.00000000 * !moread %moinp "orca.gbw" %casscf nel 7 norb 6 mult 6, 4, 2 nroots 3, 3, 3 NEVPT2 SC end Moreinputfilescanbefoundinthedirecory(onchem400a): ~nooijen/Orca_examples_2017_nlogn/NEVPT2/NArOThatisitfornow.