Ab Initio Calculation

Abstract

Ab initio calculations rest on solving the Schrödinger equation; the nature of the necessary approximations determines the level of the calculation. In the simplest approach, the Hartree-Fock method, the total molecular wavefunction $Ψ$ is approximated as a Slater determinant composed of occupied spin orbitals. To use these in practical calculations the spatial orbitals are approximated as a linear combination (a weighted sum) of basis functions. Electron correlation methods are also discussed. The main uses of the ab initio method are calculating molecular geometries, energies, vibrational frequencies, spectra, ionization energies and electron affinities, and properties like dipole moments which are connected with electron distribution. These calculations find theoretical and practical applications, since, for example, enzyme-substrate interactions depend on shapes and charge distributions, reaction equilibria and rates depend on energy differences, and spectroscopy plays an important role in identifying and understanding novel molecules. The visualization of calculated phenomena can be very important in interpreting results.