5.6.1 Introduction

Whereas RSH functionals such as LRC-$\omega$PBE are attempts to add 100% LR Hartree-Fock exchange with minimal perturbation to the original functional (PBE, in this example), other RSH functionals are of a more empirical nature and their range-separation parameters have been carefully parameterized along with all of the other parameters in the functional. These cases are functionals are discussed first, in Section 5.6.2, because their range-separation parameters should be taken as fixed. User-defined values of the range-separation parameter are discussed in Section 5.6.3, and Section 5.6.4 discusses a procedure for which an optimal, system-specific value of this parameter ($\omega$ or $\mu$) can be chosen for functionals such as LRC-$\omega$PBE or LRC-$\mu$PBE.