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.
Read more about this topic: Electronic Density
Famous quotes containing the word response:
“There are situations in life to which the only satisfactory response is a physically violent one. If you don’t make that response, you continually relive the unresolved situation over and over in your life.”
—Russell Hoban (b. 1925)