The direct and semi-direct integral transformation algorithms used by Q-Chem (e.g., MP2, CIS(D)) are limited by available disk space, , and memory, , the number of basis functions, , the number of virtual orbitals, and the number of occupied orbitals, , as discussed above. The generic description of the key $rem variables are:
MEM_STATIC
MEM_STATIC
Sets the memory for Fortran AO integral calculation and transformation modules.
TYPE:
INTEGER
DEFAULT:
64
corresponding to 64 MB.
OPTIONS:
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
MEM_TOTAL
Sets the total memory available to Q-Chem, in megabytes.
TYPE:
INTEGER
DEFAULT:
8000
Corresponding to 8000 MB.
OPTIONS:
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
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