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Quantum chemistry, also known as molecular quantum mechanics, is a division of chemistry that employs quantum mechanics to the study of chemical systems to mathematically describe the fundamental properties of atoms and molecules.
At the level of atoms and sub-atomic particles, objects behave very differently to how they might behave normally; quantum theory is an attempt to describe the behavior of matter and energy in this sub-atomic state. Quantum chemistry enables scientists to understand matter at this most fundamental level by using quantum mechanics in physical models and experiments of chemical systems.
Quantum chemistry offers a complete knowledge of the chemical properties of a system and implies the computation of the wave function that describes the electronic structure of atoms and molecules.
There are two aspects of quantum mechanics which makes quantum chemistry different from previous models of matter;
- Wave-particle duality – the need to think of very small objects such as electrons as having characteristics of both waves and particles.
- Quantum mechanical models correctly predict that the energy of atoms and molecules is always quantized, in other words, they only have specific amounts of energy.
Quantum chemistry is a powerful tool to study the ground state of individual atoms and molecules, and the excited and transition states that arise during chemical reactions. Quantum chemical theories allow scientists to explain the structure of the Periodic Table and quantum chemical calculations allow them to accurately predict the structures of molecules and the spectroscopic behavior of atoms. It can be employed to understand, model and forecast molecular properties and their reactions, properties of nanometer materials, and reactions and processes occurring in biological systems.
Schrödinger and Theoretical Quantum Chemistry
In 1925, Erwin Schrödinger investigated what an electron might look like as a wave-particle around the nucleus of an atom. The result was an equation for particle waves, which now acts as a starting point for the quantum mechanical study of the properties of atoms and molecules.
Theoretical quantum chemistry aims to calculate predictions of quantum theory as atoms and molecules can only have discrete energies. Chemists employ the Schrödinger equation to determine the allowed energy levels of quantum mechanical systems and solving the equation usually the first phase of solving a quantum chemical problem with the result inferring the chemical properties of the material.
However, a precise solution for the Schrödinger equation can only be calculated for hydrogen; because all other atomic, or molecular systems, involve three or more particles, their Schrödinger equations cannot be solved exactly and so estimated solutions are given.
Quantum Chemistry Methods
There are two commonly used methods to solve Schrödinger’s equation – ab initio and semi-empirical methods.
- Ab initio: A solution to the equation is obtained from the first principles of quantum chemistry using rigorous mathematical approximations and without using empirical methods. It utilizes two strategies to solve the equation: the first is wavefunction based, and the second is density functional-based, which involves the study of the properties of the system through its electronic density, but avoids the explicit resolve of the electronic wavefunction.
- Semi-empirical methods: these are less accurate and use experimental results to avoid the solution of some terms that appear in ab initio methods.
Experimental Quantum Chemistry
Experimental quantum chemists rely heavily on spectroscopy – IR spectroscopy, NMR spectroscopy, and scanning probe microscopy – to obtain information about the quantization of energy on a molecular scale. It has great value in supporting and interpreting experimental spectroscopic data. A close collaboration between theoretical calculations and experiments has produced many chances for quantum chemistry calculations to classify species found in spectra and to propose new avenues for experimental study.
Sources and Further Reading