Advantages of the RI approximation:

For RI-CC2 calculations, the operation count and thereby the CPU and the wall time increases--as for RI-MP2 calculations--approximately with ${\cal O}(O^2V^2N_x)$ mathend000#, where $O$ mathend000# is the number of occupied and $V$ mathend000# the number of virtual orbitals and $N_x$ mathend000# the dimension of the auxiliary basis set for the resolution of the identity. Since RI-CC2 calculations require the (iterative) solution of the cluster equations (9.5) and (9.6), they are about 10-20 times more expensive than MP2 calculations. The disk space requirements are approximately $O(2V+N)N_x + N_x^2$ mathend000# double precision words. The details of the algorithms see ref. [10], for the error introduced by the RI approximation see refs. [81,84].