# 20: Response Theory

- Page ID
- 60577

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Many physical properties of a molecule can be calculated as expectation values of a corresponding quantum mechanical operator. The evaluation of other properties can be formulated in terms of the "response" (i.e., derivative) of the electronic energy with respect to the application of an external field perturbation.

- 20.1: Calculations of Properties Other Than the Energy
- Essentially all experimentally measured properties can be thought of as arising through the response of the system to some externally applied perturbation or disturbance. In turn, the calculation of such properties can be formulated in terms of the response of the energy E or wavefunction Ψ to a perturbation.

- 20.2: Ab Initio, Semi-Empirical, and Empirical Force Field Methods
- Semi-empirical methods use experimental data or the results of ab initio calculations to determine some of the matrix elements or integrals needed to carry out their procedures. Each of these tools has advantages and limitations. Ab initio methods involve intensive computation and therefore tend to be limited to smaller atoms, molecules, radicals, and ions. Semi-empirical or empirical methods would be of little use on systems whose electronic properties have not been included in the data base.

*Thumbnail: Potential energy diagram. Image used with permission (Public Domain; Chem540grp1f08)*

## Contributors and Attributions

Jack Simons (Henry Eyring Scientist and Professor of Chemistry, U. Utah) Telluride Schools on Theoretical Chemistry and Jeff A. Nichols (Oak Ridge National Laboratory)