Quantum Chemistry in Molecular Modeling: Our Agenda

1. Quantum Chemistry inMolecular Modeling: Our Agenda • Postulates, Schrödinger equation & examples (Ch. 2-8) • Computational chemistry (Ch. 16) • Hydrogen-like atom (one-electron atom) (Ch. 9) • Many-electron atoms (Ch. 10-11) • Diatomic molecules (Ch. 12-13) • Polyatomic molecules (Ch. 14) • Solids

2. Computational Chemistry • References (on-line) • Computational chemistry: Introduction to the theory and applications of • molecular and quantum mechanics, E. Lewars (Kluwer Academic, 2004) • Chapter 5 (& 4) http://site.ebrary.com/lib/kwangju/Doc?id=10067383 • LCAO-MO: Hartree-Fock-Roothaan-Hall equation • C. C. J. Roothaan, Rev. Mod. Phys. 23, 69 (1951) • http://prola.aps.org/pdf/RMP/v23/i2/p69_1 • EMSL Basis Set Exchange (Basis Set Order Form) • http://gnode2.pnl.gov/bse/portal • Basis Sets Lab Activity • http://www.shodor.org/chemviz/basis/teachers/background.html

3. : A set of L preset basis functions Basis Set to Expand Atomic or Molecular Orbitals (complete if ) • Larger basis set give higher-quality wave functions. • (but more computationally-demanding) • H-atom orbitals • Slater type orbitals (STO; Slater) • Gaussian type orbitals (GTO; Boys) • Numerical basis functions

4. STO Basis Functions • Correct cusp behavior (finite derivative) at r  0 • Desired exponential decay at r  • Correctly mimic the H atom orbitals • Would be more natural choice • No analytic method to evaluate the coulomb and XC (or exchange) integrals GTO Basis Functions • Wrong cusp behavior (zero slope) at r  0 • Wrong decay behavior (too rapid) at r  • Analytic evaluation of the coulomb and XC (or exchange) integrals • (The product of the gaussian "primitives" is another gaussian.)

6. (not orthogonal but normalized)   or  above Smaller for Bigger shell (1s<2sp<3spd)

7. Contracted Gaussian Functions (CGF) • The product of the gaussian "primitives" is another gaussian. • Integrals are easily calculated. Computational advantage • The price we pay is loss of accuracy. • To compensate for this loss, we combine GTOs. • By adding several GTOs, you get a good approximation of the STO. • The more GTOs we combine, the more accurate the result. • STO-nG (n: the number of GTOs combined to approximate the STO) STO GTO primitive Minimal CGF basis set

8. Extended Basis Set: Split Valence • * minimal basis sets (STO-3G) • A single CGF for each AO up to valence electrons • Double-Zeta (: STO exponent) Basis Sets (DZ) • Inert core orbitals: with a single CGF (STO-3G, STO-6G, etc) • Valence orbitals: with a double set of CGFs • Pople’s 3-21G, 6-31G, etc. • Triple-Zeta Basis Sets (TZ) • Inert core orbitals: with a single CGF • Valence orbitals: with a triple set of CGFs • Pople’s 6-311G, etc.

9. Double-Zeta Basis Set: Carbon 2s Example 3 for 1s (core) 21 for 2sp (valence)

10. Basis Set Comparison

11. Double-Zeta Basis Set: Example 3 for 1s (core) 21 for 2sp (valence) Not so good agreement

12. Triple-Zeta Basis Set: Example 6 for 1s (core) 311 for 2sp (valence) better agreement

13. Extended Basis Set: Polarization Function • Functions of higher angular momentum than those occupied in the atom • p-functions for H-He, • d-functions for Li-Ca • f-functions for transition metal elements

14. Extended Basis Set: Polarization Function • The orbitals can distort and adapt better to the molecular environment. • (Example) Double-Zeta Polarization (DZP) or Split-Valence Polarization (SVP) • 6-31G(d,p) = 6-31G**, 6-31G(d) = 6-31G* (Pople)

15. Polarization Functions. Good for Geometries

16. wave function Extended Basis Set: Diffuse Function • Core electrons and electrons engaged in bonding are tightly bound. •  Basis sets usually concentrate on the inner shell electrons. • (The tail of wave function is not really a factor in calculations.) • In anions and in excited states, loosely bond electrons become important. • (The tail of wave function is now important.) •  We supplement with diffuse functions • (which has very small exponents to represent the tail). • + when added to H • ++ when added to others

17. Dunning’s Correlation-Consistent Basis Set • Augmented with functions with even higher angular momentum • cc-pVDZ (correlation-consistent polarized valence double zeta) • cc-pVTZ (triple zeta) • cc-pVQZ (quadruple zeta) • cc-pV5Z (quintuple zeta) (14s8p4d3f2g1h)/[6s5p4d3f2g1h] Basis Set Sizes

18. Effective Core Potentials (ECP) or Pseudo-potentials • From about the third row of the periodic table (K-) • Large number of electrons slows down the calculation. • Extra electrons are mostly core electrons. • A minimal representation will be adequate. • Replace the core electrons with analytic functions • (added to the Fock operator) representing • the combined nuclear-electronic core to the valence electrons. • Relativistic effect (the masses of the inner electrons of heavy atoms are • significantly greater than the electron rest mass) is taken into account by • relativistic ECP. • Hay and Wadt (ECP and optimized basis set) from Los Alamos (LANL)

20. Accuracy of ab Initio Quantum Chemistry Methods

21. Lower Level of Quantum Chemical Calculations

23. ab Initio or DFT Quantum Chemistry Softwares • Gaussian • Jaguar (http://www.schrodinger.com): Manuals on website • Turbomole • DGauss • DeMon • GAMESS • ADF (STO basis sets) • DMol (Numerical basis sets) • VASP (periodic, solid state, Plane wave basis sets) • PWSCF (periodic, solid state, Plane wave basis sets) • etc.

24. Major Three Inputs in Quantum Chemistry Calculations

25. Input #1. Geometry or Nuclear Coordinates {ZA, RA} • Cartesian coordinates {xA, yA, zA} • Z-matrix (internal coordinates) {rA, A, A} * Fractional coordinates (in crystals) {xA/a, yA/b, zA/c}

28. Input #2. Molecule and Basis Set

33. Input #4. Properties to Calculate