Nonlinear phenomena in time-dependent density-functional theory: What Rabi oscillations can teach us

JOHANNA I FUKS; NICOLE HELBIG; ILYA V. TOKATLY; ANGEL RUBIO

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: New York; Año: 2011 vol. 84

1098-0121

Through the exact solution of a two-electron singlet system interacting with a monochromatic laser we provethat all adiabatic density functionals within time-dependent density-functional theory are not able to discernbetween resonant and nonresonant (detuned) Rabi oscillations. This is rationalized in terms of a fictitiousdynamical exchange-correlation (xc) detuning of the resonance while the laser is acting. The nonlinear dynamicsof the Kohn-Sham system shows the characteristic features of detuned Rabi oscillations even if the exact resonantfrequency is used. We identify the source of this error in a contribution from the xc functional to the set ofnonlinear equations that describes the electron dynamics in an effective two-level system. The constraint ofpreventing the detuning introduces a new strong condition to be satisfied by approximate xc functionals.