Q-Chem 4.4 User’s Manual

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



Contains the molecular coordinate input (input file requisite).


Job specification and customization parameters (input file requisite).


User-defined basis set information (Chapter 7).


Options for the constrained DFT method (Section 4.10).


Job control for the Q-Chem/ChemSol interface (Langevin dipoles


model; Section 11.2.9).


User comments for inclusion into output file.


Contains parameters for complex-scaled and CAP-augmented EOM-CC


calculations (Chapter 6.7).


User-defined effective core potentials (Chapter 8).


Application of external constraints in a geometry optimization (Section 9.3.6).


Specifies labels and positions of EFP fragments (Section 11.5).


Contains user-defined parameters for effective fragments (Section 11.5).


User-defined van der Waals parameters for DFT dispersion correction




User-defined guess for EOM-CC calculations (Chapter 6.7).


Specifies external point charges and their positions.


Force-field parameters for QM/MM calculations (Section 11.3).


Intracule parameters (Section 10.9).


Isotopic substitutions for vibrational calculations (Section 10.11.6).


Information for mixing together multiple adiabatic states into diabatic


states (Chapter 10).


Details of an external multipole field (Section 3.5.7).


Options for the Natural Bond Orbital package (Section 10.3).


Guess orbitals to be occupied (Section 4.5.4).


Constraint definitions for geometry optimizations (Section 9.3).


Job control for polarizable continuum models (Section 11.2.3).


Generate plotting information over a grid of points (Section 10.5).


Information for quasi-classical trajectory calculations (Section 9.7.5).






Specify the QM region for QM/MM calculations (Section 11.3).


Additional parameters and variables for implicit solvent models


(Section 11.2).


Job control for SM$x$ implicit solvent models (Section 11.2.7).


Guess orbitals to be swapped (Section 4.5.4).


Special parameters for the iso-density SS(V)PE module (Section 11.2.5).


Initial guess for the iso-density SS(V)PE module (Section 11.2.5).


Additional parameters for two-photon absorption calculations




User-defined atomic radii for Langevin dipoles solvation (Section 11.2.9)


and PCMs (Section 11.2.2).


User-defined DFT exchange-correlation functional (Section

Table 3.1: A list of Q-Chem input sections; the first two ($molecule and $rem) are required for all jobs, whereas the rest are required only for certain job types, or else are optional places to specify additional job-control variables. Each input section (“$whatever”) 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:

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.