# X-Ray Absorption from δ-Kick

This tutorial shows how to run a Real-Time Time-Dependent DFT calculation using the so-called δ-kick approach to compute absorption electronic spectra.

This tutorial is structured as follows:

• Brief theory overview

• CP2K input file overview

Then, the link between the CP2K output (time-dependent dipole moment) and the quantity targetted (the absorption spectra in the frequency domain) is performed in this jupyter notebook: download this file. Of course, one can use its own script and method. Yet, as these analyses present some subtilities, we propose our which shows quantitative results with the Linear Response one.

## Theory Overview

### Response theory reminders

In the perturbative regime and at the linear order, the time-dependent response of an electronic cloud can be decomposed in the Fourier space: the response at a given frequency is proportional to the perturbation acting on the system at the same frequency. In particular, the induced dipole moment (or equivalently the induced current) at a given frequency, $$\mu^\omega$$, is given by the product of the polarizability matrix, $$\alpha(\omega, \omega)$$, with the perturbative electric field at the same frequency, $$F^\omega$$, within the dipolar approximation.

$\mu^\omega = \alpha(\omega, \omega) \cdot F^\omega$

This quantity involved a dot product between the field vector and the polarizability matrix. For the next paragraphs, we will drop the vectorial behavior and discuss it later.

In Linear-Response-TDDFT approaches, one computes the excited state promoted by an electric field oscillating at a specific frequency $$\omega$$ and then computes the transition dipole moment associated with such electronic transition. This transition dipole moment is then used to compute the polarizability for resonant or non-resonant frequency.

In the Real-Time approach presented here, we will excite all the possible electronic transitions at the beginning of the simulation and then propagate this excited electronic cloud to observe the induced dipole moment. We will deduce the polarizability tensor for any frequency from this induced dipole moment.

### The δ-kick technic

In the δ-kick technic, the electronic structure is first obtained at the ground state, for instance using DFT, and then perturbed with an instantaneous electric field named a δ-kick:

$F(t) = F^0 \delta(t)$

This δ-kick perturbes the ground state wave-function at $$t=0^-$$ using either the dipole moment operator (length gauge) or using the momentum one (velocity gauge). Then, this excited wave-function is propagated in real time by numerically integrating the DFT equivalent of the Schrodinger equation. For more information, have a look for instance at the recent work in CP2K by Mattiat2022.

The electric δ-kick can be written in the frequency domain:

$F^\omega = \frac{F^0}{2 \pi} \int_{-\infty}^{+ \infty} \delta(t) e^{i \omega t} dt = \frac{F^0}{2 \pi} e^{i \omega \times 0} = \frac{F^0}{2 \pi}$

The field amplitude in the Fourier space is $$F^0 / 2 \pi$$ for all frequencies: this instantaneous perturbation does indeed contain all the frequencies. The time-dependent wave-function can be thus described to be the one at the ground state plus all the possible excited states. During the time propagation, the dipole moment of the molecule will be given by the superposition of all possible oscillations related to all the excited states. This complex behavior in the time domain became simple in the frequency one since we know the amplitude of the perturbation applied at each frequency:

$\mu^\omega = \frac{1}{2 \pi} \alpha(\omega, \omega) F^0$

Therefore, in order to get the polarizability $$\alpha(\omega, \omega)$$ one prepares the perturbed state at $$t=0^-$$ for a given field amplitude, then propagates the wave-function in real time and compute the time-dependent dipole moment. The dipole moment is Fourier transformed and the polarizability is extracted using:

$\begin{split} \text{Re} \left[ \alpha(\omega, \omega) \right] = 2 \pi \frac{\text{Re} \left[ \mu^\omega \right] }{F^0} \\ \text{Im} \left[ \alpha(\omega, \omega) \right] = 2 \pi \frac{\text{Im} \left[ \mu^\omega \right] }{F^0} \end{split}$

The amplitude of the dipole moment in the Fourier space may be very small if the frequencies are far from resonances. But, in principle, one can extract the whole spectra from one Real-Time calculation. Of course, numerically speaking this is not the case since the time scale involved for UV frequencies is quite different from the one involved for X-Rays: a Real-Time propagation calculation is thus run for a specific range of frequency.

### Absorption spectrum

Using the time-dependent dipole moment, we got the frequency-dependent polarizability $$\alpha$$ assuming a perturbative and linear response regime. The real and imaginary part of the polarizability defines the nature of the response. If the frequency is at an electronic resonance, then the polarizability has an imaginary part. The polarizability is pure real off resonances.

From one Real-Time propagation, one can obtain 3 components of the polarizability tensor. For instance, applying a δ-kick along the $$x$$ direction will provides the components $$\alpha_{xx}$$, $$\alpha_{yx}$$ and $$\alpha_{zx}$$ by Fourier Transform the dipole moment along the $$x$$, $$y$$ and $$z$$ direction respectively. Therefore, to get the full polarizability tensor, one should run 3 Real-Time propagations using a perturbation along 3 orthogonal directions, for instance, $$x$$, $$y$$, and $$z$$.

To compare with experiments, one often assumes that the system is averaged over all possible orientations in space. Then, the absorption spectrum $$I(\omega)$$ is proportional to:

$I(\omega) \propto \sum_{i=x,y,z} \text{Im} \left[ \alpha_{ii}(\omega, \omega) \right]$

## CP2K Input

In this tutorial, we will study the response of a carbon-monoxide in the gas phase upon a δ-kick along its perpendicular direction. We will focus on the X-Ray range, but in principle, other ranges of frequency can be sampled using this technic.

This has been done for isolated carbon-monoxide at the DFT/PBEh level using a PCSEG-2 basis set. One can look at the input file contained in RTP.inp, in the following we will emphasize a few important points related to the δ-kick technic.

&GLOBAL
PROJECT RTP
RUN_TYPE RT_PROPAGATION
&END GLOBAL

&MOTION
&MD
ENSEMBLE NVE
STEPS 50000
TIMESTEP [fs] 0.00078
TEMPERATURE [K] 0.0
&END MD
&END MOTION

&FORCE_EVAL
METHOD QS
&DFT
&REAL_TIME_PROPAGATION
APPLY_DELTA_PULSE .TRUE.
DELTA_PULSE_DIRECTION 1 0 0
DELTA_PULSE_SCALE 0.001
MAX_ITER 100
MAT_EXP ARNOLDI
EPS_ITER 1.0E-11
INITIAL_WFN SCF_WFN
PERIODIC .FALSE.
&END REAL_TIME_PROPAGATION
BASIS_SET_FILE_NAME BASIS_PCSEG2
POTENTIAL_FILE_NAME POTENTIAL
&MGRID
CUTOFF 1000
NGRIDS 5
REL_CUTOFF 60
&END MGRID
&QS
METHOD GAPW
EPS_FIT 1.0E-6
&END QS
&SCF
MAX_SCF 500
SCF_GUESS RESTART
EPS_SCF 1.0E-8
&END SCF
&POISSON
POISSON_SOLVER WAVELET
PERIODIC NONE
&END POISSON
&XC
&XC_FUNCTIONAL PBE
&PBE
SCALE_X 0.55
&END
&END XC_FUNCTIONAL
&HF
FRACTION 0.45
&INTERACTION_POTENTIAL
POTENTIAL_TYPE TRUNCATED
&END INTERACTION_POTENTIAL
&END HF
&END XC
&PRINT
&MULLIKEN OFF
&END MULLIKEN
&HIRSHFELD OFF
&END HIRSHFELD
&MOMENTS
PERIODIC .FALSE.
FILENAME =dipole
COMMON_ITERATION_LEVELS 100000
&EACH
MD 1
&END EACH
&END MOMENTS
&END PRINT
&END DFT

&SUBSYS
&CELL
ABC 10 10 10
ALPHA_BETA_GAMMA 90 90 90
PERIODIC NONE
&END CELL
&TOPOLOGY
COORD_FILE_NAME carbon-monoxide_opt.xyz
COORD_FILE_FORMAT XYZ
&END TOPOLOGY
&KIND C
BASIS_SET pcseg-2
POTENTIAL ALL
&END KIND
&KIND O
BASIS_SET pcseg-2
POTENTIAL ALL
&END KIND
&END SUBSYS
&END FORCE_EVAL


### Time step

The time step used to propagate the wave-function is related to the frequency of interest. Typically, one should do a trade-off between the maximal frequency that can be sampled, this gives the time step value, and the accuracy of the sampling itself, related to the total simulation time. Typically, the time step should be about one order of magnitude lower than the frequency of interest. Here, we will tackle the Oxygen K-edge: the electronic transition between the Oxygen 1s and the first available excited state.

According to Linear Response TDDFT calculation (using the XAS_TDP module, see this tutorial), the first transition occurs at 529 eV with an oscillator strength of 0.044 a.u. In our δ-kick approach, it means that the induced dipole moment should have an oscillatory component around the frequency corresponding to 529 eV. Hence, we should have a time step smaller than this frequency. For this tutorial, we have set the TIMESTEP to [fs] 0.00078, which leads to about 10 time steps per period for an oscillation at 529 eV.

The total time of the simulation is then given by the number of time steps required, here 500000, and is related to the precision of the calculation: the more total time, the better the accuracy. This value is less clear to determine: it depends on the strength of the perturbation, the error thresholds used for the Real Time simulation, the amplitude of the polarizability value itself, and the overall noise of the calculation. A good way to check that the total simulation time is enough is to compute the polarizability for the targetted frequency using either 80% of the simulation time or 100%.

### Field properties

The field perturbation (the δ-kick) is defined by its amplitude and polarization.

The amplitude depends on the system of interest, typically $$10^{-3}$$ would do, and a good way to check it is to run 2 simulations: one with once the amplitude and another with twice the amplitude. Within the linear regime, the response of the system should have doubled as well. If this is not the case, lower the field amplitude. For instance, by one or two orders of magnitude. Note that applying a too-low field may lead to numerical noise. For isolated carbon monoxide, we have checked that $$10^{-3}$$ is within the linear regime.

Please note that the actual perturbation applied in CP2K is not DELTA_PULSE_SCALE and depends on the cell size. Look at the output file just before the Real Time propagation part to know the exact perturbation amplitude used

The polarization of the field determines which excited state is triggered: the electric field and the transition dipole moment should be non-perpendicular. One way to know how to select the polarization is to run a Linear-Response TDDFT calculation and to look at the transition dipole moment decomposition along the laboratory axis (or the oscillator strength decomposition). For instance, have a look at the first part of this tutorial on Real Time TDDFT with a time-dependent field for isolated carbon monoxide. Relying on this calculation, the electronic transition at 529 eV is perpendicular to the CO bond. For the geometry we are using, it means that the field polarization should be along x (or y) by setting DELTA_PULSE_DIRECTION to 1 0 0 and DELTA_PULSE_SCALE to 0.001.

### Note about the choice of the Gauge

For an isolated system, use the length implementation because the perturbation is applied up to all perturbative orders by setting PERIODIC to .FALSE..

For condensed phase systems, use the velocity gauge instead by setting PERIODIC to .TRUE.. In this case, the perturbation will be applied only within the first order. We have used the length one for this tutorial.

## Analyzing the results

To analyze the Real Time propagation simulation, we propose to go through a jupyter notebook: download this file. Note that this zip file also contains the data needed to perform the analyses (the output of the CP2K run).