Below is a brief notes on Quantum chemistry, which would be helpful in recalling the development and significance of Quantum chemistry.
Classical mechanics was developed to explain the motion and related consequences of large bodies. But it failed to explain the same when applied to the elementary particles. So, there was a need to develop a new set of laws which could explain the motion and energy associated with such elementary particles. This task is successfully accomplished by the Quantum mechanics. The Quantum chemistry is a branch of chemistry, which is related to the study of motion and energy of fundamental particles. This is a hybrid of Physics and Chemistry.
The quantum chemistry essentially began with the discovery of cathode rays by Michael Faraday, the statement of the Black body radiation problem, the suggestion by Boltzmann that the energy states of a physical system could be discrete and the quantum hypothesis by Max Planck notes back to 1900 says that the energy radiated by an atomic system can theoretically be divided into a number of discrete energy elements E, such that each of these energy elements is proportional to the frequency ν with which they each individually radiate energy, as defined by the following formula:
E = nhν
where, h is a numerical constant called Planck’s constant. Then, in 1905, to explain the Photoelectric effect, showed that light itself consists of individual quantum particles, which later came to be called Photons.
The first step in solving a quantum chemical problem starts with solving the Schrodinger wave equation, with the electronic molecular Hamiltonian. This determines the electronic structure of the molecule. The electronic structure of a crystal or molecule implies its chemical properties. An exact solution for the Schrödinger equation can be obtained for the hydrogen atom only. Since other atomic or molecular systems, involve the motions of many "particles", their Schrödinger equations cannot be solved easily.
In quantum mechanics, the physical state of a particle can be expressed as the sum of two operators, one corresponding to Kinetic energy and the other to Potential energy. The Hamiltonian in the Schrodinger equation used in quantum chemistry does not contain the terms for the spin of the electron.
Solutions of the Schrödinger equation for the hydrogen atom give the form of the wave function for atomic orbitals, and the relative energy of the various orbitals. The orbital approximation is used to understand the atomic orbitals of other atoms.