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## Foundation of MO Theory, Huckel Theory, Hartree-Fock Theory

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**Foundation of MO Theory, Huckel Theory, Hartree-Fock Theory**Lecture 4 08.3.4CH418 Computational Chemistry KAIST**Comp. Chemistry: Chemical Modeling Lab**• Overall Goal: Use mathematical and computer models to understand and predict chemical structure, properties, and reactivity • Methods: • WebMO/Gaussian/GaussView: Use existing, state-of-the-art computer models to calculate molecular properties • Mathcad(optional, use your own): Develop your own models for thermodynamic quantities, reactivity, and kinetics**Computational Chemistry**• Computer-based calculation of chemical structure, properties, and reactivity • Usefulness • Complements and explains experimental results • Goes where experiment cannot (transition states, intermediates) • Makes predictions and can guide experiments**Computational Chemistry (con’t)**• History • Past: Mainframe computers (limited to a few specialists due to difficult interface) • Present: Desktop workstations (still inaccessible to many due to system requirements, cost, and licensing) and PC • Evolving: WWW (readily available to all chemists)**WebMO Quick Start**• WebMO: spin.kaist.ac.kr/webmo/login.cgi • Login: Username=guest, Password = guest • Job Manager: Create New Job • Build Molecule: Open Editor, Build HFCO, Close Editor • Choose Engine: Gaussian • Job Options: Single Point, Hartree-Fock, Basic, Preview Input File • Preview Gaussian Input File: Submit Job • Job Manager: View**Chemical Models**• Plastic models for organic chemistry structures • Lewis structures and electron pushing for organic reactions • Computational chemistry models for structure and reactivity**Computational Chemistry Approaches**• Molecular Mechanics • Classical mechanics • Parameters kr, r0, kq, q0, ... chosen to fit observed data • No explicit treatment of electrons • Very fast • Need to specify bonding**Computational Chemistry Approaches (con’t)**• Electronic Structure Methods • Quantum Mechanics • Electrons (molecular orbitals) explicitly calculated • Much slower, but more general**Electronic Structure Methods**• Semi-empirical (MOPAC, AMPAC, HyperChem) • use parameters to evaluate integrals • relatively fast • ab initio (Gaussian, Spartan, GAMESS) • evaluate integrals from first principles • slow**Electronic Structure Methods (con’t)**• Density Functional Theory (Gaussian, GAMESS) • similar to ab initio • includes electron correlation • electron density calculated, not orbitals • not as slow**Model Chemistry(QM)**• Methods • Hartree-Fock (HF), Møller-Plesset (MP2), B3LYP • Basis Set • STO-3G, 3-21G, 6-31G(d), ... • Open vs. Closed Shell • unrestricted (U) if unpaired electrons exist • restricted (default) when all electrons are paired • Compound Methods • geometry at lower theory; energy at higher theory**Running Calculations**• WebMO User Interface • Build molecule – Submit job • Choose engine – Monitor progress • Select job options – View results • WebMO behind-the-scenes actions • Create input file • Queue and run job • Format output file • Most can be done more conveniently by GaussView • Use WebMo from remote site**Gaussian Input File**Route (job options) blank line Title blank line Charge and Multiplicity Geometry Specification #N HF/3-21G SP HFCO 0 1 C O 1 1.50 F 1 1.49 2 120.0 H 1 1.09 2 120.0 3 180.0**Z-Matrix**C O 1 1.50 F 1 1.49 2 120.0 H 1 1.09 2 120.0 3 180.0**Z-Matrix (con’t)**• Z-Matrix is chemically intuitive (atom distance, bond angle, dihedral angle) • Z-Matrix is efficient because it has only 3N-6 coordinates (vs. 3N for Cartesian coordinates) • Many possible Z-matrices due to different ordering of atoms • Near linear molecules have poorly defined dihedral angles**Gaussian Output File**• Geometry Standard orientation: • Energy SCF Done: E(RHF) = • Molecular Orbitals and Energies (Pop=Reg) Molecular Orbital Coefficients EIGENVALUES**Gaussian Output File (con’t)**• Atomic Charges Total atomic charges: • Dipole Moment Dipole moment (Debye): Tot = • NMR Shifts GIAO Magnetic shielding tensor (ppm): C Isotopic =**WebMO**• Easier input creation, job management, and result viewing • Project is stable, but always under development • About 200 international downloads to date • We want and value your feedback!!! www.webmo.net**Quantum Mechanics: Solving Schrödinger Equation (SE)**• Solving SE yields energies and wave functions of the system – any property can be obtained (most of chemistry can be discussed) • SE is defined once positions of atomic nuclei and number of electrons are given (any molecules can be handled) • Exact solutions are usually impossible, but reasonably accurate approximate solutions of SE are possible for atoms, molecules, and crystals • Ab initio quantum chemistry for molecules • First-principle electronic structure for solids • Model Hamiltonian > semi-empirical, DFT(?)