# General aspects#

In this section we will cover general aspects of electronic structure theory calculations.

Orbitals and basis sets

An introduction to wave functions for single-electron systems is given.Many-electron wave functions

An introduction to many-electron wave functions is given.Operators and matrix elements

An introduction to one- and two-electron operators is given together with a presentation of their associated matrix elements.Reduced particle densities

One- and two-particle reduced densities are presented and illustrated.Natural orbitals

Natural orbitals for restricted and unrestricted calculations are introduced.Integrals

A presentation of a technique to transform integrals in the AO basis to the MO basis as needed for correlated wave function methods.Electron spin

Definitions of spin operators and a visual illustration of collinear and noncollinear spin densities.Electron correlation

Definition of correlation energy and a visual exploration of the effects of electron correlation on one- and two-particle densities.Dispersion interactions

Definition of interaction energy and visual illustration of its association with electron correlation. Dispersion correction by means of \(C_6\) coefficients is demonstrated.Rayleigh–Schrödinger perturbation theory

A presentation of the perturbation theory in a form useful for Møller–Plesset methods.Second quantization

A presentation of the occupation number formalism is given.Group theory

A presentation of group theory its application in chemistry is given.