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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}$(O2V2Nx), where O is the number of occupied and V the number of virtual orbitals and Nx 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)Nx + Nx2 double precision words. The details of the algorithms are described in ref. [10], for the error introduced by the RI approximation see refs. [92,111].



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