title "Water TD-PBE0 resonant excitation"
echo
scratch_dir ./scratch
permanent_dir ./perm

start water

##
## aug-cc-pvtz / pbe0 optimized
##
## Note: you are required to explicitly name the geometry
##
geometry "system" units angstroms nocenter noautoz noautosym
  O     0.00000000    -0.00001441    -0.34824012
  H    -0.00000000     0.76001092    -0.93285191
  H     0.00000000    -0.75999650    -0.93290797
end

## Note: We need to explicitly set the "active" geometry even though there is only one geom.
set geometry "system"

## All DFT and basis parameters are inherited by the RT-TDDFT code
basis
 * library 6-31G
end

dft
 xc pbe0
end

## Compute ground state of the system
task dft energy

##
## We excite the system with a quasi-monochromatic
## (Gaussian-enveloped) z-polarized E-field tuned to a transition at
## 10.25 eV.  The envelope takes the form:
##
## G(t) = exp(-(t-t0)^ / 2s^2)
##
## The target excitation has an energy (frequency) of w = 0.3768 au
## and thus an oscillation period of T = 2 pi / w = 16.68 au
##
## Since we are doing a Gaussian envelope in time, we will get a
## Gaussian envelope in frequency (Gaussians are eigenfunctions of a
## Fourier transform), with width equal to the inverse of the width in
## time.  Say, we want a Gaussian in frequency with FWHM = 1 eV
## (recall FWHM = 2 sqrt (2ln2) s_freq) we want an s_freq = 0.42 eV =
## 0.0154 au, thus in time we need s_time = 1 / s_time = 64.8 au.
##
## Now we want the envelope to be effectively zero at t=0, say 1e-8
## (otherwise we get "windowing" effects).  Reordering G(t):
##
## t0 = t - sqrt(-2 s^2 ln G(t))
##
## That means our Gaussian needs to be centered at t0 = 393.3 au.
##
## The total simulation time will be 1000 au to leave lots of time to
## see oscillations after the field has passed.
##
rt_tddft
  tmax 1000.0
  dt 0.2

  field "driver"
    type gaussian
    polarization z
    frequency 0.3768  # = 10.25 eV
    center 393.3
    width 64.8
    max 0.0001
  end

  excite "system" with "driver"
 end
task dft rt_tddft