HS 2013: Vorlesung Physikalische Chemie III Part 2 Electronic Structure Theory Lecture 6 Dr. Mar'n O. Steinhauser Fraunhofer Ins'tute for High- Speed Dynamics, Ernst- Mach- Ins'tut, EMI, Freiburg Email: mar'n.steinhauser@unibas.ch or mar'n.steinhauser@emi.fraunhofer.de Web: hkp://www.chemie.unibas.ch/~steinhauser 1 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 1
Contents 1. Introduction 2. The Electronic Schrödinger Equation 3. The Hartree-Fock Approximation 4. Configuration Interaction 5. Perturbation Theory 6. Density functional Theory 2 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 2
Suggested Literature Chemistry Related: A. Szabo, N. S. Ostlund, Modern Quantum Chemistry, Mineola, NY, Dover Publica'ons, 1996 F. Jensen, Introduc4on to Computa4onal Chemistry 2nd ed., New York, John Wiley & Sons, 2007 N. Ira Levine, Quantum Chemistry 5th ed., Upper Saddle River, NJ, Pren'ce Hall, 1999 W. Koch, M. C. Holthausen, A Chemist s Guide to Density Func4onal Theory, Weinheim, Wiley- VCH, 2001 Quantum Theory: C. Cohen- Tannoudji, B. Diu, F. Laloë, Quantum Mechanics, New York, John Wiley & Sons, 1977 R. G. Parr, W. Yang, Density- Func4onal Theory of Atoms and Molecules, New York, Oxford University Press, 1989 J. J. Sakurai, Modern Quantum Mechanics, Addison- Wesley, 1994 P. A. M. Dirac, Lectures on Quantum Mechanics, Mineola, New York, Dover Publica'ons, 2001 Computer Simula'on and Modeling: M. P. Allen, D. J. Tildesley, Computer Simula4on of Liquids, Clarendon Press, Oxford, 1987 D. C. Rappaport, The Art of Molecular Dynamics Simula4on, University Press, Cambridge, 1995 J. M. Thijssen, Computa4onal Physics, Cambridge New York, Melbourne, Cambridge University Press, 1999 M. O. Steinhauser, Mul4scale Modeling of Fluids and Solids Theory and Applica4ons, Springer, Heidelberg, Berlin, New York, 2008 Kurt Binder, D. W. Heermann, Monte- Carlo Simula4ons in Sta4s4cal Physics, Heidelberg, London, New York, Springer 2010 3 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 3
Problems With Using a Single Slater Determinant Let us consider some spin- orbital product (Slater determinant) wave func'ons for two electrons in π and π * orbitals. Examples of binding (π) and an'bining (π * ) orbitals Let us consider two electrons in their π orbitals (e.g. in an olefin) when we break the π- bond into di- radicals by twis'ng it by 90 degrees. 4 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 4
Problems With Using a Single Slater Determinant! Singlet!!π 2 :!!det πα ( 1)πβ ( 2) = 2 1 2 π ( 1)π ( 2) α 1 ( )β 2! Singlet!!π *2 :!!det π * α ( 1)π * β ( 2) = 2 1 2 π * ( 1)π * ( 2) α 1! Triplet!!ππ * :!!det πα ( 1)π * α 2 ( )π * β 2 det πβ 1! ( ) = 2 1 2 π 1 ( ) = 2 1 2 π 1 2 1 2 det πα ( 1)π * β ( 2) + πβ ( 1)π * α ( 2) = 2 1 π ( 1)π * ( 2)α ( 1)β 2!!!!!!!!π! * ( 1)π ( 2)β ( 1)α 2 Singlet!ππ * :!2 1 2 det πα ( 1)π * β 2!!!!!!!!!!!!!!!!!!!!!!!!!!!2 1 π ( 1)π * ( 2)α ( 1)β 2!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!π! * ( 1)π ( 2)β ( 1)α 2 ( ) β 1 ( )β 2 ( )α 2 ( ) ( ) β 1 5 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 5 ( )α 2 ( ) ( )π * ( 2) π ( 2)π * ( 1) α ( 1)α 2 ( ) ( )π * ( 2) π ( 2)π * ( 1) β ( 1)β 2 ( ) + π 1 ( ) + π * 1 ( ) πβ 1 ( )π * ( 2)β ( 1)α 2 ( ) ( )π ( 2)α ( 1)β ( 2) ( )π * α 2 ( ) π 1 ( ) + π * 1 ( ) = ( )π * ( 2)β ( 1)α 2 ( ) ( )π ( 2)α ( 1)β ( 2) ( )
Problems With Using a Single Slater Determinant Now!think!of!π = 2 1/2! to!consider!the!bahavior!when!π 3bond!cleavage!occurs!upon!rotation.! Singlet!!π 2 :!!det πα ( 1)πβ ( 2) = 2 1 det Rα ( 1)Rβ ( 2)! + det Lα ( 1)Lβ ( 2)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ionic + diradical + det Rα ( 1)Lβ ( 2)! + det Lα ( 1)Rβ ( 2)!! Triplet!!ππ * :!! diradical!!!!!!!!!!!!!!!!!!!!!!!!det πα ( 1)π * α 2!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Singlet((ππ * : ((try(to(calculate(yourself! (((((((((((((((((((((((( ionic ( L + R)!and!π * = 2 1/2 ( L R)!!! Singlet!!π *2 :!!det π * α ( 1)π * β ( 2) = 2 1 det Rα ( 1)Rβ ( 2)! + det Lα ( 1)Lβ ( 2)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ionic + diradical det Rα ( 1)Lβ ( 2)! det Lα ( 1)Rβ ( 2)!! ( ) = 2 1 det Lα 1 (((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((( ( )Rα ( 2)! det Rα ( 1)Lα ( 2)! Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 6 6
Problems With Using a Single Slater Determinant So,!the!spin!state!and!orbital!occupancy!plus!the!antisymmetry!have!! effects!on!the!ionic/radical!character!of!the!wave!function.! To!adequatly!dexcribe!the!π 2!bond!breaking,!we!need!to!mix π 2!und!π *2!configuration!state!functions!(CSF).!Single!configuration! functions!and!single!determinants!are!not!always!adequate! 7 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 7
Restricted vs. Unrestricted Hartree- Fock (RHF vs. UHF) What kind of approxima'ons can we do further? Restricted (RHF): Electrons with different spin occupy the same orbital part There is more flexibility by assuming: Electrons don t need to be paired Unrestricted (UHF): Electrons with different spin can occupy different orbitals 8 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 8
Dissocia'on of H 2 FCI 9 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 9
What is Actually Missing in the Hartree- Fock Equa'ons? Correla'ons (missing by defini'on!) " Dynamical Correla'on (electrons get too close to each other in mean- field approxima'on) " Sta'c Correla'on (single determinant approxima'on is not good enough to describe bond breaking) Possible Solu'on to this?! Configuration!Interaction!(CI) 10 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 10
Equa'ons with Constraints: Lagrangian Mul'pliers Here, we want to consider the general method of solving equa'ons which have certain constraints by using Lagrangian Mul'pliers λ. For example, we derived the HF- Equa'ons using: L { χ } i { } ( ) = E HF χ i λ ij i j δ ij General Func'onal We actually minimize the energy func'onal, i.e. the expecta'on value of the Hamiltonian by varying { } χ i This is our constraint: We demand the func'ons to be orthonormal { } χ i Lagrange Mul'plier(s) (one for each constraint) Whiteboard Notes 11 Lecture HS 2013 - University of Basel: Electronic Structure Theory (PC III) 11