MaxDisk Keyword
Last Update: 12/31/2000

 Description

The MaxDisk keyword specifies the amount of disk storage available for scratch data, in 8-byte words.The value may optionally be followed by a units designation: KB, MB, GB, KW, MW or GB. Normally, this is set for a site in the site-wide Default.Rou file.

MP3, MP4, QCISD, CCSD, QCISD(T)< and CCSD(T) calculations all now look at MaxDisk. If the calculation can be done using a full integral transformation while keeping disk usage under MaxDisk, this is done; if not, a partial transformation is done and some terms are computed in the AO basis. Since MP2 obeys MaxDisk as much as possible, the Stingy, NoStingy and VeryStingy options are not needed.

Thus, it is crucial for a value for MaxDisk to be specified explicitly for these types of jobs, either within the route section or via a system wide setting in the Default.Rou file. If MaxDisk is left unset, the program now assumes that disk is abundant and performs a full transformation by default, in contrast to Gaussian 94 wher a partial transformation was the default in such cases. If MaxDisk is not set and sufficient disk space is not available for a full transformation, the job will fail (where it may have worked in G94).

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 if possible, but have substantial 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.

  • CID, CISD, CCD, BD, and QCISD densities, and CCSD, CCSD(T), QCISD(T), and BD(T) energies have fixed disk requirements proportional to ON3 which is used cannot be limited by MaxDisk.

  • CID, CISD, CCD, and QCISD gradients have fixed disk requirements of about N4/2 for closed-shell and 3N4/4 for open-shell.

  • Storage of AO integrals is not affected by MaxDisk; any calculations which might consume all of the available disk should be run using SCF=Direct (the default setting) to avoid consuming limited disk space with AO integrals.

See chapter 4 for a detailed discussion of the efficient use of disk resources in Gaussian 98 calculations.

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