Here we are concerned with a very simple strategy to reduce computational cost of singlereference correlated calculations, that is, truncating the virtual orbital space while leaving the occupied space untouched, such that the exact correlation energy of a given method is smoothly recovered as the fraction of frozen virtual orbitals approaches zero. By using perturbation theory arguments, one can, of course, justify simply freezing high-lying canonical Hartree-Fock orbitals. However, natural occupations provide even better gauge of the relative importance of orbitals in terms of their contributions into the total correlation energy. In order to preserve the definition of the vacuum (which is determined by choosing a particular reference determinant), the concept of Frozen Natural Orbitals (FNOs) was introduced . FNOs are eigenstates of the virtual-virtual block of the one-particle density matrix; thus, they can be used to transform only the virtual space, without changing the reference determinant. Of course, to compute FNOs one needs to know the full correlated wave function. A typical strategy of using FNOs as a mean to reduce computational costs is to use a density matrix computed at a lower level of theory (say, MP2, which scales as