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# 2.1.3 Memory and Disk Requirements

(February 4, 2022)

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 6.16. 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 about 1.4GB 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.