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, KW, MW or GW (without intervening spaces) to specify units of kilo-, mega- or giga- bytes or words. Normally, this is set for a site in the site-wide 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.
See Efficiency Considerations for a detailed discussion of the efficient use of disk resources in Gaussian calculations.
Last updated on: 10 May 2009