Computational Chemistry A branch of chemistry concerned with

Computational Chemistry: • A branch of chemistry concerned with theoretically determining properties of molecules. • Use mathematical approximations and computer programs to obtain results relative to chemical problems.

Computational Chemistry: • Because of the difficulty of dealing with some materials, computational modeling has become an important characterization tool in chemical laboratory. • predict new molecules or new reactions which are later investigated experimentally.

Computational results: • • • Geometrical structures (bond length and bond angle). Vibrational energy levels (IR and Raman spectra) Electronic energy levels (UV and visible spectra) Thermochemistry (ΔH and ΔG) Potential energy curve (barrier energy, transition states. Electron affinities (LUMO energy). Ionization energy (HOMO energy). IR and Raman intensities. Dipole moments. Polarizabilities. Magnetic shielding tensors (NMR spectra).

Schrödinger’s Equation: Hamiltonian oberator Iegen value (total energy) Hψ = Eψ Iegen function (wave function) • gives a complete description of the electronic structure of a molecule and Describes both the wave and particle behavior of electrons.

Schrödinger’s Equation: Hamiltonian oberator Iegen value (total energy) Hψ = Eψ Iegen function (wave function) • As solutions can be obtained only for very simple systems, we need for approximations to treat the larger molecules.

Computational Methods: Semi Empirical: Uses experimental parameters and extensive simplifications of Schrödinger's equation. It is less accurate but much faster Ab initio: Uses Schrödinger's equation, but with approximations e. g. Hartree fock (HF) method Density Functional Theory (DFT): Considered an ab initio method, but different because the wavefunction is not used to describe a molecule, instead the electron density is used.

Basis Set: Functions used to describe the shape of the orbitals in a molecule, each basis set is a different group of constants used in the wavefunction of the Schrödinger equation.