# Time-dependent Hartree-Fock and density functional theory¶

*Modules*: `tdscf`

, `pbc.tdscf`

## Introduction¶

PySCF implements the time-dependent Hartree-Fock (TDHF) and
time-dependent density functional theory (TDDFT) (frequency domain)
linear response theories to compute excited-state energies
and transition properties in the `tdscf`

module.
A minimal example that runs a TDDFT calculation is as follows

```
from pyscf import gto, scf, dft, tddft
mol = gto.Mole()
mol.build(
atom = 'H 0 0 0; F 0 0 1.1', # in Angstrom
basis = '631g',
symmetry = True,
verbose = 4,
)
mf = dft.RKS(mol)
mf.xc = 'b3lyp'
mf.kernel()
mytd = tddft.TDDFT(mf)
mytd.nstates = 10
mytd.kernel()
mytd.analyze()
```

The example above computes the excitation energies, oscillator strengths and transition dipole moments of the ten lowest singlet exicted states.

## Theory¶

Using first-order time-dependent perturbation theory within HF or KS theory, one obtains the non-Hermitian TDHF or TDDFT equations for the excitation energies [1]:

where \(\mathbf{A}\) and \(\mathbf{B}\) are the orbital hessians which also appear in the stability analysis for reference states (see Stability analysis), \(\omega\) is the excitation energy, and \(\mathbf{X}\) and \(\mathbf{Y}\) represent the response of the density matrix. In cases where the system possesses a degenerate ground state or has triplet instabilities, the algorithms used to solve the above equations may be unstable. This can be solved by applying the Tamm-Dancoff approximation (TDA) [2], which simply neglects the \(\mathbf{B}\) and \(\mathbf{Y}\) matrices and leads to a Hermitian eigenvalue problem

## Methods¶

For TDHF or TDDFT calculations, the reference state can be either restricted or unrestricted:

```
mytd = mol.RKS().run().TDDFT().run()
mytd = mol.UKS().run().TDDFT().run()
```

By default, only singlet excited states are computed.
In order to compute triplet excited states, one needs to set the
attribute `singlet`

to `False`

:

```
mytd.singlet = False
mytd.kernel()
```

One can also perform symmetry analysis by calling the `analyze()`

method,
which also computes the oscillator strengths and dipole moments:

```
mytd.analyze(verbose=4)
```

## Property calculation¶

### Oscillator strengths¶

Oscillator strengths for each excited state can be computed in both length and velocity gauges:

```
mytd.oscillator_strength(gauge='length')
mytd.oscillator_strength(gauge='velocity')
```

Higher order corrections [3] to the oscillator strength can also be included:

```
#include corrections due to magnetic dipole and electric quadruple
mytd.oscillator_strength(gauge='velocity', order=1)
#also include corrections due to magnetic quadruple and electric octupole
mytd.oscillator_strength(gauge='velocity', order=2)
```

### Transition moments¶

PySCF implements various types of transition moments between the reference SCF state and the TDHF or TDDFT excited states. These include:

electric dipole, quadrupole and octupole transition moments in both length and velocity gauges:

mytd.transition_dipole() mytd.transition_velocity_dipole() mytd.transition_quadrupole() mytd.transition_velocity_quadrupole() mytd.transition_octupole() mytd.transition_velocity_octupole()

magnetic dipole and quadrupole transition moments:

mytd.transition_magnetic_dipole() mytd.transition_magnetic_quadrupole()

### Nuclear gradients¶

Analytic nuclear gradients are available for TDHF and TDDFT, and they can be computed as follows:

```
tdg = mytd.Gradients()
g1 = tdg.kernel() #default will compute the gradients of first excited state
g1 = tdg.kernel(state=1) #first excited state
g2 = tdg.kernel(state=2) #second excited state
```

### Natural transition orbital analysis¶

Natural transition orbitals (NTOs) can be computed by singular value decomposition of the transition density matrix. In PySCF, these orbitals can be obtained as follows:

```
weights, nto_coeff = mytd.get_nto(state=1)
```

where `nto_coeff`

are the coefficients for NTOs represented in AO basis,
and they are ordered as occupied orbitals followed by virtual orbitals.

## References¶

- 1
Andreas Dreuw and Martin Head-Gordon. Single-reference ab initio methods for the calculation of excited states of large molecules.

*Chemical reviews*, 105(11):4009–4037, 2005.- 2
So Hirata and Martin Head-Gordon. Time-dependent density functional theory within the tamm–dancoff approximation.

*Chemical Physics Letters*, 314(3-4):291–299, 1999.- 3
Patrick J Lestrange, Franco Egidi, and Xiaosong Li. The consequences of improperly describing oscillator strengths beyond the electric dipole approximation.

*The Journal of chemical physics*, 143(23):234103, 2015.