# prefactor_opa¶

psi4.driver.p4util.prefactor_opa()[source]

Prefactor for converting microscopic observable to decadic molar extinction coefficient in one-photon absorption.

Notes

This function implements the calculation of the following prefactor:

$k = \frac{4\pi^{2}N_{\mathrm{A}}}{3\times 1000\times \ln(10) (4 \pi \epsilon_{0}) n \hbar c}$

The prefactor is computed in SI units and then adjusted for the fact that we use atomic units to express microscopic observables: excitation energies and transition dipole moments. The refractive index $$n$$ is, in general, frequency-dependent. We assume it to be constant and equal to 1.

Return type:

float