Response

Name TDDeltaKickTime
Section Time-Dependent::Response
Type float
Default 0.0

The delta-perturbation that can be applied by making use of the TDDeltaStrength variable, can be applied at the time given by this variable. Usually, this time is zero, since one wants to apply the delta pertubation or "kick" at a system at equilibrium, and no other time-dependent external potential is used. However, one may want to apply a kick on top of a laser field, for example.

Name TDDeltaStrength
Section Time-Dependent::Response
Type float
Default 0

When no laser is applied, a delta (in time) perturbation with strength TDDeltaStrength can be applied. This is used to calculate, e.g., the linear optical spectra. If the ions are allowed to move, the kick will affect them also. The electric field is $-(\hbar k / e) \delta(t)$ for a dipole with zero wavevector, where k = TDDeltaStrength, which causes the wavefunctions instantaneously to acquire a phase $e^{ikx}$. The unit is inverse length.

Name TDDeltaStrengthMode
Section Time-Dependent::Response
Type integer
Default kick_density

When calculating the density response via real-time propagation, one needs to perform an initial kick on the KS system, at time zero. Depending on what kind of response property one wants to obtain, this kick may be done in several modes. For use to calculate triplet excitations, see MJT Oliveira, A Castro, MAL Marques, and A Rubio, J. Nanoscience and Nanotechnology 8, 3392 (2008).

Options:

• kick_density:
  The total density of the system is perturbed. This mode is appropriate for electric dipole response, as for optical absorption.
  
• kick_spin:
  The individual spin densities are perturbed oppositely. Note that this mode is only possible if the run is done in spin-polarized mode, or with spinors. This mode is appropriate for the paramagnetic dipole response, which can couple to triplet excitations.
  
• kick_spin_and_density:
  A combination of the two above. Note that this mode is only possible if the run is done in spin-polarized mode, or with spinors. This mode is intended for use with symmetries to obtain both of the responses at once, at described in the reference above.
  
• kick_magnon:
  Rotates the magnetization. Only works for spinors. Can be used in a supercell or my making use of the generalized Bloch theorem. In the later case (see SpiralBoundaryConditions) spin-orbit coupling cannot be used.
  

Name TDDeltaUserDefined
Section Time-Dependent::Response
Type string

By default, the kick function will be a dipole. This will change if (1) the variable TDDeltaUserDefined is present in the inp file, or (2) if the block TDKickFunction is present in the inp file. If both are present in the inp file, the TDKickFunction block will be ignored. The value of TDDeltaUserDefined should be a string describing the function that is going to be used as delta perturbation.

Name TDKickFunction
Section Time-Dependent::Response
Type block

If the block TDKickFunction is present in the input file, and the variable TDDeltaUserDefined is not present in the input file, the kick function to be applied at time zero of the time-propagation will not be a "dipole" function (i.e. $\phi \rightarrow e^{ikx} \phi$, but a general multipole in the form $r^l Y_{lm}(r)$.

Each line has three columns: integers l and m that defines the multipole, and a weight. Any number of lines may be given, and the kick will be the sum of those multipoles with the given weights.

This feature allows calculation of quadrupole, octupole, etc., response functions.

Name TDMomentumTransfer
Section Time-Dependent::Response
Type block

Momentum-transfer vector for the calculation of the dynamic structure factor. When this variable is set, a non-dipole field is applied, and an output file ftchd is created (it contains the Fourier transform of the charge density at each time). The type of the applied external field can be set by an optional last number. Possible options are qexp (default), qcos, qsin, or qcos+qsin. In the formulae below, $\vec{q}$ is the momentum-transfer vector.

Options:

• qexp:
  External field is $e^{i \vec{q} \cdot \vec{r}}$.
  
• qcos:
  External field is $\cos \left( i \vec{q} \cdot \vec{r} \right)$.
  
• qsin:
  External field is $\sin \left( i \vec{q} \cdot \vec{r} \right)$.
  
• qbessel:
  External field is $j_l \left( \vec{q} \cdot \vec{r} \right) Y_{lm} \left(\vec{r} \right)$. In this case, the block has to include two extra values (l and m).
  

Name TDMultipleMomentumTransfer
Section Time-Dependent::Response
Type block

For magnon kicks only. A simple way to specify momentum-transfer vectors for the calculation of the magnetization dynamics. This variable should be used for a supercell. For each reciprocal lattice vectors, the code will kick the original magnetization using all the multiples of it. The syntax reads:

%TDMultipleMomentumTransfer
  N_x | N_y | N_z
%

and will include the (2N_x+1)(2N_y+1)(2N_z+1) multiples vectors of the reciprocal lattice vectors of the current cell.

Name TDReducedMomentumTransfer
Section Time-Dependent::Response
Type block

The same as TDMomentumTransfer, but the momentum is specified in reduced coordinates. Only available for magnon kicks at the moment, and only with an exponential kick.