Linear response

Polarizability¶

The linear electric-dipole polarizabilty is determined from the linear response function Norman et al. (2018)

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
task: response
@end

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

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

@molecule
charge: 0
multiplicity: 1
xyz:  
...
@end 

General linear response functions¶

A general linear response function

can be requested, referring to the linear response of the molecular property associated with $\hat{\Omega}$ due to the perturbation associated with $\hat{V}$ and oscillating with the angular frequency $\omega$. The damping term $\gamma$ is associated with the inverse lifetime of the excited states.

Python script

import veloxchem as vlx

molecule = vlx.Molecule.read_xyz_string("""4
Hydrogen peroxide
O  -0.65564532 -0.06106286 -0.03621403
O   0.65564532  0.06106286 -0.03621403
H  -0.97628735  0.65082652  0.57474201
H   0.97628735 -0.65082652  0.57474201
""")

basis = vlx.MolecularBasis.read(molecule, "def2-svpd", ostream=None)

scf_drv = vlx.ScfRestrictedDriver()

scf_drv.xcfun = "b3lyp"
scf_results = scf_drv.compute(molecule, basis)

lrf = vlx.ComplexResponse() 

# available operators
#lrf.b_operator = "electric dipole"
#lrf.b_operator = "magnetic dipole"
#lrf.b_operator = "linear momentum"
#lrf.b_operator = "angular momentum"

lrf.a_operator = "electric dipole"
lrf.b_operator = "magnetic dipole"

lrf.a_components = ["x", "y", "z"]
lrf.b_components = ["x", "y", "z"]

lrf_drv.damping = 0.004556  # 1000 cm-1
lrf.frequencies = [0.0656]

lrf_results = lrf.compute(molecule, basis, scf_results)

C6 dispersion coefficients¶

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

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

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 Norman et al. (2018).

@jobs
task: response
@end

@method settings
xcfun: b3lyp
grid_level: 4
basis: def2-svpd
@end

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

@molecule
charge: 0
multiplicity: 1
xyz:
...
@end

Laser pulse propagation¶

VeloxChem allows for calculation of the linear response to a Gaussian-envelope pulse.

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 plain text 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. Note in particular that the default of carrier envelope phase may need adjustment to match your desired setup.

@jobs
task: pulses
@end

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

@molecule
charge: 0
multiplicity: 1
xyz:
...
@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.