  We analyze the role of the difference between the central frequencies of the
spectral distributions of the vector potential and the electric field of a
short laser pulse. The frequency shift arises when the electric field is
determined as the derivative of the vector potential to ensure that both
quantities vanish at the beginning and end of the pulse. We derive an
analytical estimate of the frequency shift and show how it affects various
light induced processes, such as excitation, ionization and high harmonic
generation. Since observables depend on the frequency spectrum of the electric
field, the shift should be taken into account when setting the central
frequency of the vector potential to avoid potential misinterpretation of
numerical results for processes induced by few-cycle pulses.
