Electric Field Dependence of the Exchange-Correlation Potential in Molecular Chains

Density functional calculations on the (non)linear optical properties of conjugated molecular chains using currently popular exchange-correlation (xc) potentials give overestimations of several orders of magnitude. By analyzing ``exact'' and Krieger-Li-Iafrate xc potentials, the error is traced back to an incorrect electric field dependence of the ``response part'' of the xc potential in local and gradient-corrected density approximations, which lack a linear term counteracting the applied electric field.