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In difference to CC2 and MP2, the CCSD model does no longer allow
to avoid the storage of double excitation amplitudes (*t*_{aibj}
)
and intermediates of with a similar size. Thus, also the disc space
requirements for the CCSD calculation are larger than for
RI-MP2 and RI-CC2 calculation for the same system.
For a (closed-shell) CCSD ground state energy calculations the amount of
disc space needed can be estimated roughly as
*N*_{disk} 4*N*^{3} + (4 + *m*_{DIIS})*O*^{2}*N*^{2}/(128×1024) *MBytes* , |
(10.24) |

where *N*
is the number of basis functions, *O*
the number of occupied
orbitals and *m*_{DIIS}
the number of vectors used in the DIIS
procedure (by default 10, see Sec. 18.2.14 for details).
For (closed-shell) CCSD(T) calculations the required disc space
is with

*N*_{disk} 5*N*^{3} +5*O*^{2}*N*^{2} + *ON*^{3}/(128×1024) MBytes , |
(10.25) |

somewhat larger.
For calculations with an open-shell (UHF or ROHF) reference wavefunctions
the above estimates should be multiplied by factor of 4.

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