The electronic density corresponding to a normalized -electron wavefunction is defined as where the operator corresponding to the density observable is Computing as defined above we can simplify the expression as follows. In words: holding a single electron still in position we sum over all possible arrangements of the other electrons. In Hartree–Fock and density functionaltheoriesthe wave function is typically represented as a single Slater determinant constructed from orbitals,, with corresponding occupations. In these situations the density simplifies to
General Properties
From its definition, the electron density is a non-negative function integrating to the total number of electrons. Further, for a system with kinetic energyT, the density satisfies the inequalities For finite kinetic energies, the first inequality places the square root of the density in the Sobolev space. Together with the normalization and non-negativity this defines a space containing physically acceptable densities as The second inequality places the density in the L3 space. Together with the normalization property places acceptable densities within the intersection of L1 and L3 – a superset of.
The electronic density displays cusps at each nucleus in a molecule as a result of the unbounded electron-nucleus Coulomb potential. This behavior is quantified by the Kato cusp condition formulated in terms of the spherically averaged density,, about any given nucleus as That is, the radial derivative of the spherically averaged density, evaluated at any nucleus, is equal to twice the density at that nucleus multiplied by the negative of the atomic number.
Asymptotic behavior
The nuclear cusp condition provides the near-nuclear density behavior as The long-range behavior of the density is also known, taking the form where I is the ionization energy of the system.
Response Density
Another more-general definition of a density is the "linear-response density". This is the density that when contracted with any spin-free, one-electron operator yields the associated property defined as the derivative of the energy. For example, a dipole moment is the derivative of the energy with respect to an external magnetic field and is not the expectation value of the operator over the wavefunction. For some theories they are the same when the wavefunction is converged. The occupation numbers are not limited to the range of zero to two, and therefore sometimes even the response density can be negative in certain regions of space.