Paratec has an option to apply an external (electric) potential of the form:
This is done with an entry like:
electric_potential_kvec 1 0 0 1
The last number switches the field on (=1) or off (=0), the first 3 INTEGER numbers specify the wave number in reciprocal lattice coordinates.
The amplitude (in Rydbergs) is entered as:
electric_potential_strength 0.3