# 7.3.1 Brief Introduction to TDDFT

Excited states may be obtained from density functional theory by time-dependent density functional theory,152, 238 which calculates poles in the response of the ground state density to a time-varying applied electric field. These poles are Bohr frequencies, or in other words the excitation energies. Operationally, this involves solution of an eigenvalue equation

 $\left(\begin{array}[]{cc}\mathbf{A}&\mathbf{B}\\ \mathbf{B}^{\dagger}&\mathbf{A}^{\dagger}\\ \end{array}\right)\left(\begin{array}[]{c}\mathbf{x}\\ \mathbf{y}\end{array}\right)=\omega\left(\begin{array}[]{cc}\mathbf{-1}&% \mathbf{0}\\ \mathbf{0}&\mathbf{1}\\ \end{array}\right)\left(\begin{array}[]{c}\mathbf{x}\\ \mathbf{y}\end{array}\right)$ (7.15)

where the elements of the matrix $\mathbf{A}$ similar to those used at the CIS level, Eq. (7.11), but with an exchange-correlation correction.377 Elements of $\mathbf{B}$ are similar. Equation (7.15) is solved iteratively for the lowest few excitation energies, $\omega$. Alternatively, one can make a CIS-like Tamm-Dancoff approximation (TDA),378 in which the “de-excitation” amplitudes $\mathbf{Y}$ are neglected, the $\mathbf{B}$ matrix is not required, and Eq. (7.15) reduces to $\mathbf{Ax}=\omega\mathbf{x}$.

TDDFT is popular because its computational cost is roughly similar to that of the simple CIS method, but a description of differential electron correlation effects is implicit in the method. It is advisable to only employ TDDFT for low-lying valence excited states that are below the first ionization potential of the molecule,152 or more conservatively, below the first Rydberg state, and in such cases the valence excitation energies are often remarkably improved relative to CIS, with an accuracy of $\sim$0.3 eV for many functionals.531 The calculation of the nuclear gradients of full TDDFT and within the TDA is implemented.575

On the other hand, standard density functionals do not yield a potential with the correct long-range Coulomb tail, owing to the so-called self-interaction problem, and therefore excitation energies corresponding to states that sample this tail (e.g., diffuse Rydberg states and some charge transfer excited states) are not given accurately.150, 919, 516 The extent to which a particular excited state is characterized by charge transfer can be assessed using an a spatial overlap metric proposed by Peach, Benfield, Helgaker, and Tozer (PBHT).700 (However, see Ref. 782 for a cautionary note regarding this metric.)

Standard TDDFT also does not yield a good description of static correlation effects (see Section 6.10), because it is based on a single reference configuration of Kohn-Sham orbitals. Recently, a new variation of TDDFT called spin-flip (SF) DFT was developed by Yihan Shao, Martin Head-Gordon and Anna Krylov to address this issue.828 SF-DFT is different from standard TDDFT in two ways:

• The reference is a high-spin triplet (quartet) for a system with an even (odd) number of electrons;

• One electron is spin-flipped from an alpha Kohn-Sham orbital to a beta orbital during the excitation.

SF-DFT can describe the ground state as well as a few low-lying excited states, and has been applied to bond-breaking processes, and di- and tri-radicals with degenerate or near-degenerate frontier orbitals. Recently, we also implemented83 a SF-DFT method with a non-collinear exchange-correlation potential from Tom Ziegler et al.,957, 825 which is in many case an improvement over collinear SF-DFT.828 Recommended functionals for SF-DFT calculations are 5050 and PBE50 (see Ref. 83 for extensive benchmarks). See also Section 7.8.3 for details on wave function-based spin-flip models.