Quantum chemistry in Molecular Modeling

Fred Brouwer

[Chapter 3 of Computational Chemistry and Organic Synthesis by Hens Borkent]

                                                      Contents

5.1 Why use Quantum Chemical methods ?: Quantum-chemical methods are more

general than empirical methods. References.

5.2 The Schrodinger equation: The recipe for the calculation of the electronic wavefunction.

5.3 Solving the electronic Schrodinger equation : Hartree-Fock SCF theory:

     Part a: The independent particle approximation

     Part b: Molecular Orbitals: the LCAO MO method

                   Basis functions

                   Standard basis sets

                   Example

5.4 Limitations of the HF method:

     Electron correlation

     CI, MP2 and MCSCF

5.5 Energy calculations:

     Conversion of absolute energies to heats of formation

     Isodesmic reactions

5.6 Quality of ab initio results:

     Performance of low-level methods for organic molecules

5.7 Semi-empirical quantum chemistry:

     Further approximations and introduction of empirical parameters

5.8 Quality of semi-empirical results:

     Low-level ab initio methods are usually better, but much more time-consuming

5.9 Solvation:

     The medium can be represented as a dielectric continuum

5.10 Atomic charges:

     Not a physical concept !

     Several methods exist for attributing charge to individual atoms

"For calculating molecular properties, quantum chemistry seems to be the obvious

tool to use. Calculations that do not use the Schrödinger equation are acceptable

only to the extent that they reproduce the results of high level quantum

mechanical calculations."

(U. Burkert & N.L. Allinger, "Molecular Mechanics", 1982) 

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