The $rem section in the input file is the means by which users specify the type of calculation that they wish to perform (i.e., level of theory, basis set, convergence criteria, additional special features, etc.). The keyword $rem signals the beginning of the overall job specification. Within the $rem section the user inserts $rem variables (one per line) which define the essential details of the calculation. The allowed format is either
REM_VARIABLE VALUE [ comment ]
or alternatively
REM_VARIABLE = VALUE [ comment ]
The “=” sign is automatically discarded and only the first two remaining arguments are read, so that all remaining text is ignored and can be used to place comments in the input file. Thus the $rem section that provides Q-Chem job control takes the form shown in the following example.
Example 3.12 General format of the $rem section of the text input file.
$rem
REM_VARIABLE value [ comment ]
REM_VARIABLE value [ comment ]
...
$end
Note: (1) Tab stops can be used to format input.
(2) A line prefixed with an exclamation mark ‘!’ is treated as a comment and will be ignored by the program.
(3) $rem variables are case-insensitive (as is the whole Q-Chem input file).
(4) Depending on the particular $rem variable, “value” may be a keyword (string), an integer, or a logical value (true or false).
(5) A complete list of $rem variables can be found in Appendix C.
In this manual, $rem variables will be described using the following format:
REM_VARIABLE_NAME
A short description of what the variable controls.
TYPE:
The type of variable (INTEGER, LOGICAL or STRING)
DEFAULT:
The default value, if any.
OPTIONS:
A list of the options available to the user.
RECOMMENDATION:
A brief recommendation, where appropriate.
If a default setting is indicated for a particular $rem variable, then it is not necessary to declare that variable in order for the default setting to be used. For example, the default value for the variable JOBTYPE is SP, indicating a single-point energy calculation, so to perform such a calculation the user does not need to set the JOBTYPE variable. To perform a geometry optimization, however, it is necessary to override this default by setting JOBTYPE = OPT. System administrator preferences for default $rem settings can be specified in the $QC/config/preferences file, and user preferences in a $HOME/.qchemrc file, both of which are described in Section 2.6.
Q-Chem provides defaults for most $rem variables, but the user will always have to stipulate a few others. In a single point energy calculation, for example, the minimum requirements will be BASIS (defining the basis set) and METHOD (defining the level of theory for correlation and exchange). For example, METHOD = HF invokes a Hartree-Fock calculation, whereas METHOD = CIS specifies a CIS excited-state calculation.
Example 3.13 Example of minimal $rem requirements to run an MP2/6-31G* single-point energy calculation.
$rem
BASIS 6-31G* Just a small basis set
METHOD mp2 MP2
$end
The level of theory can alternatively be specified by setting values for two other $rem variables, EXCHANGE (defining the level of theory to treat exchange) and CORRELATION (defining the level of theory to treat electron correlation, if required). For excited states computed using equation-of-motion (EOM) methods (Chapter 7), there is a third $rem variable, EOM_CORR, which specifies the level of correlation for the target states.
For DFT calculations, METHOD specifies an exchange-correlation functional; see Section 5.4 for a list of supported functionals. For wave function approaches, supported values of METHOD can be found in Section 6.1 for ground-state methods and in Section 7.1 for excited-state methods. If a wave function-based correlation treatment such as MP2 or CC is requested using the CORRELATION keyword, then HF is taken as the default for EXCHANGE.