Q-Chem 4.3 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 3.2 and the following sections. The user must understand the function and format of the $molecule (Section 3.3) and $rem (Section 3.6) keywords, as these keyword sections are where the user places the molecular geometry information and job specification details.

The keywords $rem and $molecule are requisites of Q-Chem input files

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 start of the input file.

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 4.9).

$chem_sol

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

 

Section 11.2.8).

$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).

$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.

$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 10.11).

$isotopes

Isotopic substitutions for vibrational calculations (Section 10.13.6).

$localized_diabatization

Information for mixing together multiple adiabatic states into diabatic

 

states (Chapter 10).

$multipole_field

Details of an external multipole field.

$nbo

Options for the Natural Bond Orbital package.

$occupied

Guess orbitals to be occupied.

$opt

Constraint definitions for geometry optimizations.

$pcm

Job control for polarizable continuum models (Section 11.2.3).

$plots

Generate plotting information over a grid of points (Section 10.6).

$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.

$swap_occupied_virtual

Guess orbitals to be swapped.

$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.

$2pa

Additional parameters for 2PA calculations.

$van_der_waals

User-defined atomic radii for Langevin dipoles solvation (Section 11.2.8)

 

and PCMs (Section 11.2.2).

$xc_functional

User-defined DFT exchange-correlation functional.

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.
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.