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(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.

$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