# Linear response¶

## Polarizability¶

The linear electric-dipole polarizabilty is determined from the linear response function [NRS18]

$\alpha_{\alpha\beta}(\omega) = - \langle\langle \hat{\mu}_\alpha; \hat{\mu}_\beta \rangle \rangle_\omega$

The frequencies of the perturbing electric field is specfied as a list or in terms of a frequency region with a frequency point separation in parenthesis.

@jobs
@end

@method settings
basis: aug-cc-pvdz
@end

@response
property: polarizability
frequencies: 0-0.25 (0.05)
@end

@molecule
charge: 0
multiplicity: 1
units: au
xyz:
O   0.0   0.0   0.0
H   0.0   1.4   1.1
H   0.0  -1.4   1.1
@end


## C6 dispersion coefficients¶

The $$C_6$$ dispersion coefficient relates to the electric-dipole polarizability according to

$C_6 = \frac{3\hbar}{\pi} \int^{\infty}_0 \bar{\alpha}_A(i\omega) \bar{\alpha}_B(i\omega) d\omega$

where $$\bar{\alpha}_A{i\omega}$$ is the isotropic average of the polarizability tensor for molecular system $$A$$.

The integral over the positive imaginary frequency axis is performed in VeloxChem using a Gauss–Legendre quadrature after substituting the integration variables according to

$i\omega^I = i\omega_0 \frac{1-t}{1+t},\quad d\omega^I = \frac{-2\omega_0 dt}{(1+t)^2},$

where a transformation factor of $$\omega_0 = 0.3$$ a.u. is used. The user may specify the number of frequency points used in the quadrature, or otherwise a default value is adopted. The polarizabilities are calculated from the complex polarization propagator (CPP), or complex linear response function [NRS18].

@jobs
@end

@method settings
basis: DEF2-SVPD
dft: yes
grid_level: 4     # this is the default grid density
xcfun: b3lyp
@end

@response
property: C6
conv_thresh: 1.0e-3
n_points: 7
@end

@molecule
charge: 0
multiplicity: 1
units: ang
xyz:
C -5.8575354 3.6359158 -1.6619244
...
@end


## Laser pulse propagation¶

VeloxChem allows for calculation of the linear electric dipole response for the frequency region of a single Gaussian-envelope pulse deemed sufficiently large (using the @pulses module).

The time-domain shape parameters for this pulse may be specified by the user, optionally storing a collection of pertinent results in an HDF5-formatted file or a plaintext ASCII file whose name may be specified by the user.

An example of an input file that when run will carry out such a calculation is given below. For more documentation about the available keywords, please consult the source file whose path from the VeloxChem root folder is src/pymodule/pulsedrsp.py. Note in particular that the default of carrier envelope phase may need adjustment to match your desired setup.

@jobs
@end

@method settings
basis: aug-cc-pvdz
@end

@molecule
charge: 0
multiplicity: 1
units: au
xyz:
O   0.0   0.0   0.0
H   0.0   1.4   1.1
H   0.0  -1.4   1.1
@end

@pulses
envelope: gaussian
field_max : 1.0e-5
number_pulses: 1
centers: 300
field_cutoff_ratio: 1e-5
frequency_range : 0.2-0.4(0.001)
carrier_frequencies: 0.325
pulse_widths: 50
pol_dir: xyz
h5 : pulsed
ascii : pulsed
@end


If HDF5-formatted data was produced during this calculation, that data may used for plot generation using the script located at utils/pulsed_response_plot.py from the VeloxChem root folder.

Also note that other standard python modules such as matplotlib must be installed on the system from which this script is run. The script will take the HDF5-formatted data produced during the VeloxChem calculation and generate a plot of the real and imaginary frequency-domain electric dipole polarizability, a representation of the perturbing field in the frequency domain, the resulting (real-valued) first-order dipole moment correction in the time domain and a representation of the perturbing field in the time domain.

For more information and further description of how to run this script, please consult the documentation written inside it.