This variable can be used to enable photons in several types of calculations.
It can be used to activate the one-photon OEP formalism.
In the case of CalculationMode = casida, it enables photon modes as
described in ACS Photonics 2019, 6, 11, 2757-2778.
Finally, if set to yes when solving the ferquency-dependent Sternheimer
equation, the photons are coupled to the electronic subsystem.