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The MaxDisk keyword specifies the amount of disk storage available for scratch data, in 8-byte words (default). This value may also be followed by KB, MB, GB, TB, KW, MW, GW or TW (without intervening spaces) to specify units of kilo-, mega-, giga- or tera-bytes or words. Normally, this is set for a site in the Default.Route file.

Not all calculations can dynamically control their disk usage, so the effects of this keyword vary:

  • SCF energy, gradient, and frequency calculations use a fixed amount of disk. This is quite small (only cubic in the size of the system) and is not usually a limitation.
  • MP2 energies and gradients obey MaxDisk, which must be at least 2ON2.
  • Analytic MP2 frequencies attempt to obey MaxDisk, but have minimum disk requirements.
  • CI-Singles energies and gradients in the MO basis require about 4O2N2 words of disk for a limited set of transformed integrals. Additional scratch space is required during the transformation and this is limited as specified by MaxDisk. This disk requirement can be eliminated entirely by performing a direct CI-Singles calculation by using CIS=Direct.
  • CID, CISD, CCD, BD, and QCISD energies also have a fixed storage requirement proportional to O2N2, with a large factor, but obey MaxDisk in avoiding larger storage requirements.
  • CCSD, CCSD(T), QCISD(T), and BD(T) energies have fixed disk requirements proportional to ON3 which cannot be limited by MaxDisk.
  • CID, CISD, CCD, QCISD densities, and BD and CCSD densities and gradients have fixed disk requirements of about N4/2 for closed-shell and 3N4/4 for open-shell.
  • EOM-CCSD initially sets its space requirements to its minimum needs. If this amount is more than MaxDisk, it obeys the latter at the expense of I/O and computation time.

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