Time-dependent density functional theory

The formal foundation of TDDFT is the Runge–Gross (RG) theorem (1984)[1] – the time-dependent analogue of the Hohenberg–Kohn (HK) theorem (1964).[2] The RG theorem shows that, for a given initial wavefunction, there is a unique mapping between the time-dependent external potential of a system and its time-dependent density. This implies that the many-body wavefunction, depending upon 3N variables, is equivalent to the density, which depends upon only 3, and that all properties of a system can thus be determined from knowledge of the density alone. Unlike in DFT, there is no general minimization principle in time-dependent quantum mechanics. Consequently, the proof of the RG theorem is more involved than the HK theorem.

Given the RG theorem, the next step in developing a computationally useful method is to determine the fictitious non-interacting system which has the same density as the physical (interacting) system of interest. As in DFT, this is called the (time-dependent) Kohn–Sham system. This system is formally found as the stationary point of an action functional defined in the Keldysh formalism.[3]

The most popular application of TDDFT is in the calculation of the energies of excited states of isolated systems and, less commonly, solids. Such calculations are based on the fact that the linear response function – that is, how the electron density changes when the external potential changes – has poles at the exact excitation energies of a system. Such calculations require, in addition to the exchange-correlation potential, the exchange-correlation kernel – the functional derivative of the exchange-correlation potential with respect to the density.[4][5]

Linear-response TDDFT can be used if the external perturbation is small in the sense that it does not completely destroy the ground-state structure of the system. In this case one can analyze the linear response of the system. This is a great advantage as, to first order, the variation of the system will depend only on the ground-state wave-function so that we can simply use all the properties of DFT.

Consider a small time-dependent external perturbation ${\displaystyle \delta V^{ext}(t)}$. This gives

${\displaystyle H'(t)=H+\delta V^{ext}(t)}$

${\displaystyle H'_{KS}[\rho ](t)=H_{KS}[\rho ]+\delta V_{H}[\rho ](t)+\delta V_{xc}[\rho ](t)+\delta V^{ext}(t)}$

and looking at the linear response of the density

${\displaystyle \delta \rho (\mathbf {r} t)=\chi (\mathbf {r} t,\mathbf {r'} t')\delta V^{ext}(\mathbf {r'} t')}$

${\displaystyle \delta \rho (\mathbf {r} t)=\chi _{KS}(\mathbf {r} t,\mathbf {r'} t')\delta V^{eff}[\rho ](\mathbf {r'} t')}$

where ${\displaystyle \delta V^{eff}[\rho ](t)=\delta V^{ext}(t)+\delta V_{H}[\rho ](t)+\delta V_{xc}[\rho ](t)}$ Here and in the following it is assumed that primed variables are integrated.

Within the linear-response domain, the variation of the Hartree (H) and the exchange-correlation (xc) potential to linear order may be expanded with respect to the density variation

${\displaystyle \delta V_{H}[\rho ](\mathbf {r} )={\frac {\delta V_{H}[\rho ]}{\delta \rho }}\delta \rho ={\frac {1}{|\mathbf {r} -\mathbf {r'} |}}\delta \rho (\mathbf {r'} )}$

and

${\displaystyle \delta V_{xc}[\rho ](\mathbf {r} )={\frac {\delta V_{xc}[\rho ]}{\delta \rho }}\delta \rho =f_{xc}(\mathbf {r} t,\mathbf {r'} t')\delta \rho (\mathbf {r'} )}$

Finally, inserting this relation in the response equation for the KS system and comparing the resultant equation with the response equation for the physical system yields the Dyson equation of TDDFT:

${\displaystyle \chi (\mathbf {r} _{1}t_{1},\mathbf {r} _{2}t_{2})=\chi _{KS}(\mathbf {r_{1}} t_{1},\mathbf {r} _{2}t_{2})+\chi _{KS}(\mathbf {r_{1}} t_{1},\mathbf {r} _{2}'t_{2}')\left({\frac {1}{|\mathbf {r} _{2}'-\mathbf {r} _{1}'|}}+f_{xc}(\mathbf {r} _{2}'t_{2}',\mathbf {r} _{1}'t_{1}')\right)\chi (\mathbf {r} _{1}'t_{1}',\mathbf {r} _{2}t_{2})}$

From this last equation it is possible to derive the excitation energies of the system, as these are simply the poles of the response function.

Other linear-response approaches include the Casida formalism (an expansion in electron-hole pairs) and the Sternheimer equation (density-functional perturbation theory).

• Hohenberg, P.; Kohn, W. (1964). "Inhomogeneous Electron Gas". Physical Review. 136 (3B): B864. Bibcode:1964PhRv..136..864H. doi:10.1103/PhysRev.136.B864.
• Runge, Erich; Gross, E. K. U. (1984). "Density-Functional Theory for Time-Dependent Systems". Physical Review Letters. 52 (12): 997. Bibcode:1984PhRvL..52..997R. doi:10.1103/PhysRevLett.52.997.

• M.A.L. Marques; C.A. Ullrich; F. Nogueira; A. Rubio; K. Burke; E.K.U. Gross, eds. (2006). Time-Dependent Density Functional Theory. Springer-Verlag. ISBN 978-3-540-35422-2.
• Carsten Ullrich (2012). Time-Dependent Density-Functional Theory: Concepts and Applications. Oxford Graduate Texts. Oxford University Press. ISBN 978-0-19-956302-9.

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• tddft.org
• Brief introduction of TD-DFT