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# 2.6.1 Overview

(February 4, 2022)

Q-Chem has developed a simple mechanism for users to set user-defined long-term defaults to override the built-in program defaults. Such defaults may be most suited to machine specific features such as memory allocation, as the total available memory will vary from machine to machine depending on specific hardware and accounting configurations. However, users may identify other important uses for this customization feature. Q-Chem obtains input initialization variables from four sources:

1. 1.

User input file

2. 2.

$HOME/.qchemrc file 3. 3.$QC/config/preferences file

4. 4.

“Factory installed” program defaults

Input mechanisms higher in this list override those that are lower. Mechanisms #2 and #3 allow the user to specify alternative default settings for certain variables that will override the Q-Chem “factory-installed” defaults. This can be done by a system administrator via a preferences file added to the $QC/config directory, or by an individual user by means of a .qchemrc file in her home directory. Note: The .qchemrc and preferences files are not requisites for running Q-Chem and currently only support keywords in the$rem input section.

The format of the .qchemrc and preferences files consists of a $rem keyword section, as in the Q-Chem input file, terminated with the usual$end keyword. Any other $whatever section will be ignored. To aid in reproducibility, a copy of the .qchemrc file (if present) is included near the top of the job’s output file. (The .qchemrc and preferences files must have file permissions such that they are readable by the user invoking Q-Chem.) The format of both of these files is as follows: $rem
rem_variable   option   comment
rem_variable   option   comment
...
$end  Example 2.2.1 An example of a .qchemrc file to override default$rem settings for all of the user’s Q-Chem jobs.

$rem DIIS_SUBSPACE_SIZE 5 Modify max DIIS subspace size THRESH 10 10**(-10) threshold MAX_SCF_CYCLES 100 More than the default of 50$end


The following \$rem variables are specifically recommended as those that a user might want to customize:

• MEM_STATIC

• SCF_CONVERGENCE

• THRESH

• MAX_SCF_CYCLES

• GEOM_OPT_MAX_CYCLES