Quantum Chemistry
Quantum chemistry applies quantum mechanics to atoms and molecules, deriving electronic structure, bonding, and spectra from the Schrodinger equation and the approximations needed to solve it.
Definition
Quantum chemistry is the branch of physical chemistry that uses the principles of quantum mechanics to determine the electronic structure, energies, bonding, and properties of atoms and molecules.
Scope
This area covers the quantum-mechanical foundations of chemistry: the molecular Schrodinger equation and the wavefunction; the Born-Oppenheimer separation of electronic and nuclear motion; the construction of molecular orbitals from atomic orbitals and the resulting picture of chemical bonding; and the variational and perturbation methods, together with the Hartree-Fock and density functional approaches, used to obtain approximate solutions. The experimental probing of these structures through spectroscopy and the heavily computational implementations are treated in neighbouring areas.
Sub-topics
Core questions
- How does the Schrodinger equation describe the electrons and nuclei of a molecule?
- Why can electronic and nuclear motion be separated by the Born-Oppenheimer approximation?
- How do molecular orbitals built from atomic orbitals explain chemical bonding?
- How do variational and perturbation methods yield approximate energies and wavefunctions?
Key concepts
- Molecular Schrodinger equation and wavefunction
- Born-Oppenheimer approximation
- Molecular orbitals and chemical bonding
- Variational principle and perturbation theory
- Hartree-Fock and density functional methods
Key theories
- Molecular orbital theory
- Electrons in molecules occupy orbitals delocalized over the whole molecule, built as linear combinations of atomic orbitals; bonding and antibonding combinations and their occupation explain bond order, magnetism, and reactivity.
- Hartree-Fock self-consistent field method
- Each electron is treated as moving in the average field of the others, giving a set of coupled one-electron equations solved iteratively to self-consistency; it provides the reference from which more accurate correlated methods are built.
Clinical relevance
Quantum chemistry supplies the electronic-structure basis for predicting molecular geometries, reaction energetics, spectra, and reactivity, underpinning computational drug discovery, materials design, catalysis, and the interpretation of spectroscopic measurements.
History
Quantum chemistry began in 1927 with the Heitler-London treatment of the hydrogen molecule; valence bond theory was developed by Pauling and molecular orbital theory by Hund and Mulliken, and the Hartree-Fock method and later density functional theory turned the field into a predictive, computational discipline.
Key figures
- Erwin Schrodinger
- Linus Pauling
- Robert S. Mulliken
Related topics
Seminal works
- mcquarrie1997
- levinequantum2014
- szabo1996
Frequently asked questions
- What is the difference between valence bond and molecular orbital theory?
- Valence bond theory builds bonds from localized electron pairs shared between specific atoms, while molecular orbital theory spreads electrons over orbitals encompassing the whole molecule; both are approximations to the same quantum reality and each is more convenient for different problems.
- Why can the Schrodinger equation not be solved exactly for most molecules?
- Beyond the simplest one-electron systems the mutual repulsion between electrons couples their motions inseparably, so exact solutions are impossible and chemists rely on systematic approximations such as Hartree-Fock, perturbation theory, and density functional methods.