6.4 Exact MP2 Methods

6.4.2 Algorithm Control and Customization

(June 30, 2021)

The direct and semi-direct integral transformation algorithms used by Q-Chem (e.g., MP2, CIS(D)) are limited by available disk space, D, and memory, C, the number of basis functions, N, the number of virtual orbitals, V and the number of occupied orbitals, O, as discussed above. The generic description of the key $rem variables are:

MEM_STATIC
       Sets the memory for Fortran AO integral calculation and transformation modules.
TYPE:
       INTEGER
DEFAULT:
       64 corresponding to 64 MB.
OPTIONS:
       n User-defined number of megabytes.
RECOMMENDATION:
       For direct and semi-direct MP2 calculations, this must exceed OVN + requirements for AO integral evaluation (32–160 MB), as discussed above.

MEM_TOTAL
       Sets the total memory available to Q-Chem, in megabytes.
TYPE:
       INTEGER
DEFAULT:
       2000 Corresponding to 2000 MB.
OPTIONS:
       n User-defined number of megabytes.
RECOMMENDATION:
       Use the default, or set equal to the physical memory of your machine. Note that if the memory allocation total more than 1 GB for a CCMAN job, the memory is allocated as follows 12% MEM_STATIC 50% CC_MEMORY 35% Other memory requirements:

CD_ALGORITHM
       Determines the algorithm for MP2 integral transformations.
TYPE:
       STRING
DEFAULT:
       Program determined.
OPTIONS:
       DIRECT Uses fully direct algorithm (energies only). SEMI_DIRECT Uses disk-based semi-direct algorithm. LOCAL_OCCUPIED Alternative energy algorithm (see 6.4.1).
RECOMMENDATION:
       Semi-direct is usually most efficient, and will normally be chosen by default.

Example 6.1  Example of an MP2/6-31G* calculation employing the frozen core approximation. Note that the EXCHANGE $rem variable will default to HF

$molecule
   0 1
   O
   H1  O  oh
   H2  O  oh  H1  hoh

   oh  = 1.01
   hoh = 105
$end

$rem
   METHOD          mp2
   BASIS           6-31g*
   N_FROZEN_CORE   fc
$end

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