Disc space requirements:

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Disc space requirements:

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 $\displaystyle \approx$ $\displaystyle \Big($ 4N³ + (4 + m_DIIS)O²N² $\displaystyle \Big)$ /(128×1024) MBytes ,

(10.18)

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. 15.2.13 for details).

For (closed-shell) CCSD(T) calculations the required disc space is with

N_disk $\displaystyle \approx$ $\displaystyle \Big($ 5N³ +5O²N² + ON³ $\displaystyle \Big)$ /(128×1024) MBytes ,

(10.19)

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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