  We present an accurate and computationally efficient first-principles
methodology to calculate the natural optical activity. Our approach is based on
the long-wave density-functional perturbation theory and includes
self-consistent field (SCF) terms naturally in the formalism, which are found
to be of crucial importance. The final result is expressed exclusively in terms
of response functions to uniform field perturbations and avoids troublesome
summations over empty states. Our strategy is validated by computing the
natural optical activity tensor in representative chiral crystals (trigonal Se,
$\alpha$-HgS and $\alpha$-SiO$_2$) and molecules (C$_4$H$_4$O$_2$), finding
excellent agreement with experiment and previous theoretical calculations.
