3.2 General Form

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.

Section Name	Description
$molecule	Contains the molecular coordinate input (input file requisite).
$rem	Job specification and customization parameters (input file requisite).
$basis	User-defined basis set information (Chapter 7).
$cdft	Options for the constrained DFT method (Section 4.11).
$chem_sol	Job control for the Q-Chem/ChemSol interface (Langevin dipoles
	model; Section 11.2.9).
$comment	User comments for inclusion into output file.
$complex_ccman	Contains parameters for complex-scaled and CAP-augmented EOM-CC
	calculations (Chapter 6.7).
$ecp	User-defined effective core potentials (Chapter 8).
$efei	Application of external forces in a geometry optimization (Section 9.3.6).
$efp_fragments	Specifies labels and positions of EFP fragments (Section 11.5).
$efp_params	Contains user-defined parameters for effective fragments (Section 11.5).
$empirical_dispersion	User-defined van der Waals parameters for DFT dispersion correction
	(Section 4.4.7.2).
$eom_user_guess	User-defined guess for EOM-CC calculations (Chapter 6.7).
$external_charges	Specifies external point charges and their positions.
$force_field_params	Force-field parameters for QM/MM calculations (Section 11.3).
$intracule	Intracule parameters (Section 10.10).
$isotopes	Isotopic substitutions for vibrational calculations (Section 10.11.2).
$localized_diabatization	Information for mixing together multiple adiabatic states into diabatic
	states (Chapter 10).
$multipole_field	Details of an external multipole field (Section 3.5.7).
$nbo	Options for the Natural Bond Orbital package (Section 10.3).
$occupied	Guess orbitals to be occupied (Section 4.5.4).
$opt	Constraint definitions for geometry optimizations (Section 9.3).
$pcm	Job control for polarizable continuum models (Section 11.2.3).
$plots	Generate plotting information over a grid of points (Section 10.5).
$qct_active_modes	Information for quasi-classical trajectory calculations (Section 9.7.5).
$qct_vib_distribution
$qct_vib_phase
$qm_atoms	Specify the QM region for QM/MM calculations (Section 11.3).
$solvent	Additional parameters and variables for implicit solvent models
	(Section 11.2).
$smx	Job control for SM $x$ implicit solvent models (Section 11.2.8).
$swap_occupied_virtual	Guess orbitals to be swapped (Section 4.5.4).
$svp	Special parameters for the iso-density SS(V)PE module (Section 11.2.5).
$svpirf	Initial guess for the iso-density SS(V)PE module (Section 11.2.5).
$2pa	Additional parameters for two-photon absorption calculations
	(Section 6.7.15.1).
$van_der_waals	User-defined atomic radii for Langevin dipoles solvation (Section 11.2.9)
	and PCMs (Section 11.2.2).
$xc_functional	User-defined DFT exchange-correlation functional (Section 4.4.3.6).

Table 3.1: A list of Q-Chem input sections; the first two ($molecule and $rem) are required for all jobs, whereas the rest are required only for certain job types, or else are optional places to specify additional job-control variables. Each input section (“$section”) should be terminated with $end. See the $QC/samples directory that is included with your release for specific examples of Q-Chem input files using these keywords.

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.