# 3.2 General Form

(June 30, 2021)

IQmol (or another graphical interface) is the simplest way to control Q-Chem. However, the low level command line interface is available to enable maximum customization and allow the user to exploit all Q-Chem’s features. The command line interface requires a Q-Chem input file which is simply an ASCII text file. This input file can be created using your favorite editor (e.g., vi, emacs, jot, etc.) following the basic steps outlined in the next few chapters.

Q-Chem’s input mechanism uses a series of keywords to signal user input sections of the input file. As required, the Q-Chem program searches the input file for supported keywords. When Q-Chem finds a keyword, it then reads the section of the input file beginning at the keyword until that keyword section is terminated the $end keyword. A short description of all Q-Chem keywords is provided in Table 3.1 and the following sections. The user must understand the function and format of the$molecule (Section 3.3) and $rem (Section 3.4) keywords, as these keyword sections are where the user places the molecular geometry information and job specification details. The keywords$rem and $molecule are required in any Q-Chem input file As each keyword has a different function, the format required for specific keywords varies somewhat, to account for these differences (format requirements are summarized in Appendix C). However, because each keyword in the input file is sought out independently by the program, the overall format requirements of Q-Chem input files are much less stringent. For example, the$molecule section does not have to occur at the very beginning of the input file.

Note:  (1) Users are able to enter keyword sections in any order. (2) Each keyword section must be terminated with the $end keyword. (3) The$rem and $molecule sections must be included. (4) It is not necessary to have all keywords in an input file. (5) Each keyword section is described in Appendix C. (6) The entire Q-Chem input is case-insensitive. The second general aspect of Q-Chem input is that there are effectively four input sources: • User input file (required) • .qchemrc file in$HOME (optional)

• preferences file in $QC/config (optional) • Internal program defaults and calculation results (built-in) The order of preference is as shown, i.e., the input mechanism offers a program default override for all users, default override for individual users and, of course, the input file provided by the user overrides all defaults. Refer to Section 2.6 for details of .qchemrc and preferences. Currently, Q-Chem only supports the$rem keyword in .qchemrc and preferences files.

In general, users will need to enter variables for the $molecule and$rem keyword section and are encouraged to add a \$comment for future reference. The necessity of other keyword input will become apparent throughout the manual.