Quantum chemistry

Quantum chemistry, or molecular quantum mechanics, is a branch of physical chemistry which applies quantum mechanics to chemical systems to predict physical and chemical properties of molecules and materials. It often involves calculating electronic wave functions at the atomic level. These calculations make approximations to make simulations computationally feasible while capturing the relevant contributions to the computed wave functions and observable properties including the structures, spectra, and thermodynamics of a system. Quantum chemistry also covers the computation of quantum effects on molecular dynamics and chemical kinetics. Quantum chemistry studies focus on the electronic ground state and excited states of atoms, molecules, and ions. Such calculations allow chemical reactions to be described with respect to pathways, intermediates, and transition states. Spectroscopic properties may also be predicted. Typically, such studies assume the electronic wave function is adiabatically parameterized by the nuclear positions (i.e., the Born–Oppenheimer approximation). A wide variety of approaches are used, including semi-empirical methods, density functional theory, Hartree–Fock calculations, quantum Monte Carlo methods, and coupled cluster methods. Understanding electronic structure and molecular dynamics through the development of computational solutions to the Schrödinger equation is a central goal of quantum chemistry. Progress in the field depends on overcoming several challenges, including the need to increase the accuracy of the results for small molecular systems, as well as to increase the size of large molecules that can be realistically subjected to computation, which is limited by scaling considerations—the computation time increases as a power of the number of atoms.