next up previous contents index
Next: Required input data: Up: CC2 Ground-State Energy Calculations Previous: CC2 Ground-State Energy Calculations   Contents   Index

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)$, where $ O$ is the number of occupied and $ V$ the number of virtual orbitals and $ N_x$ 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$ double precision words. The details of the algorithms are described in ref. [10], for the error introduced by the RI approximation see refs. [85,89].