Memory
Q-Chem, Inc. has endeavored to minimize memory requirements and maximize the
efficiency of its use. Still, the larger the structure or the higher the level
of theory, the more memory is needed. Although Q-Chem can be run successfully
in very small-memory environments, this is seldom an issue nowadays and we
recommend 2 GB as a minimum.
Q-Chem also offers the ability for user control of important,
memory-intensive aspects of the program. In general, the more memory your
system has, the larger the calculation you will be able to perform.
Q-Chem uses two types of memory: a chunk of static memory that is used by multiple data sets and managed by the code, and dynamic memory which is allocated using system calls. The size of the static memory is specified by the user through the $rem variable MEM_STATIC and has a default value of 64 MB.
The $rem word MEM_TOTAL specifies the limit of the total memory the
user’s job can use. The default value is sufficiently large that on most
machines it will allow Q-Chem to use all the available memory. This value
should be reduced on machines where this is undesirable (for example if the
machine is used by multiple users). The limit for the dynamic memory allocation
is given by (MEM_TOTAL MEM_STATIC). The amount of
MEM_STATIC needed depends on the size of the user’s particular job.
Please note that one should not specify an excessively large value for
MEM_STATIC, otherwise it will reduce the available memory for dynamic
allocation. Memory settings in CC, EOM, and ADC calculations are described in
Section 6.14. The use of $rem variables will be discussed in the
next Chapter.
Disk
The Q-Chem executables, shell scripts, auxiliary files, samples and
documentation require between 360–400 MB of disk space, depending on the
platform. The default Q-Chem output, which is printed to the designated
output file, is usually only a few kilobytes. This will be exceeded, of course,
in difficult geometry optimizations, QM/MM and QM/EFP jobs, as well as in cases
where users invoke non-default print options. In order to maximize the
capabilities of your copy of Q-Chem, additional disk space is required for
scratch files created during execution, and these are automatically deleted on
normal termination of a job. The amount of disk space required for scratch
files depends critically on the type of job, the size of the molecule and the
basis set chosen.
Q-Chem uses direct methods for Hartree-Fock and density functional theory calculations, which do not require large amount of scratch disk space. Wave function-based correlation methods, such as MP2 and coupled-cluster theory require substantial amounts of temporary (scratch) disk storage, and the faster the access speeds, the better these jobs will perform. With the low cost of disk drives, it is feasible to have between 100 and 1000 GB of scratch space available as a dedicated file system for these large temporary job files. The more you have available, the larger the jobs that will be feasible and in the case of some jobs, like MP2, the jobs will also run faster as two-electron integrals are computed less often.