About Gaussian 16 Input

Syntax

Gaussian 16 input consists of a series of lines in an ASCII text file. The basic structure of a Gaussian input file includes several different sections:

• Link 0 Commands: Locate and name scratch files (not blank line terminated).
• Route section (# lines): Specify desired calculation type, model chemistry, and other options (blank line terminated). See Model Chemistries and Job Types for information about Gaussian 16 capabilities.
• Title section: Brief description of the calculation (blank line-terminated). This section is required in the input, but is not interpreted in any way by the Gaussian 16 program. It appears in the output for purposes of identification and description. Typically, this section might contain the compound name, its symmetry, the electronic state, and any other relevant information. The title section cannot exceed five lines and must be followed by a terminating blank line. The following characters should be avoided in the title section: @  #  !  –  _  \  control characters (especially Ctrl-G)
• Molecule specification: Specify molecular system to be studied (blank line-terminated). Full information is available in Molecule Specifications.
• Optional additional sections: Additional input needed for specific job types (usually blank line-terminated).

Many Gaussian 16 jobs will include only the second, third, and fourth sections. Here is an example of such a file, which requests a single point energy calculation on water:

In this job, the route and title sections each consist of a single line. The molecule specification section begins with a line giving the charge and spin multiplicity for the molecule: 0 charge (neutral molecule) and spin multiplicity 1 (singlet) in this case. The charge and spin multiplicity line is followed by lines describing the location of each atom in the molecule; this example uses Cartesian coordinates to do so. Molecule specifications are discussed in more detail later in this chapter.

The following input file illustrates the use of Link 0 commands and an additional input section:

This job requests a geometry optimization. The input section following the molecule specification is used by the Opt=ModRedundant keyword, and it serves to add an additional bond and angle in the internal coordinates used in the geometry optimization. The job also specifies a name for the checkpoint file.

For convenience, the table in the Section Ordering section details all possible sections that might appear within a Gaussian 16 input file along with the keywords associated with each one.

Syntax Rules

In general, Gaussian input is subject to the following syntax rules:

• Input is free-format and case-insensitive.
• Spaces, tabs, commas, or forward slashes can be used in any combination to separate items within a line. Multiple spaces are treated as a single delimiter.
• Options to keywords may be specified in any of the following forms:
  • keyword = option
  • keyword(option)
  • keyword=(option1, option2, …)
  • keyword(option1, option2, …)
• Multiple options are enclosed in parentheses and separated by any valid delimiter (commas are conventional and are shown above). The equals sign before the opening parenthesis may be omitted, or spaces may optionally be included before and/or after it. Note that some options also take values; in this case, the option name is followed by an equals sign: for example, CBSExtrap(NMin=6).
• All keywords and options may be shortened to their shortest unique abbreviation within the entire Gaussian 16 system. Thus, the Conventional option to the SCF keyword may be abbreviated to Conven, but not to Conv (due to the presence of the Convergence option). This holds true whether or not both Conventional and Convergence happen to be valid options for any given keyword.
• The contents of an external file may be included within a Gaussian 16 input file using the following syntax: @filename. This causes the entire file to be placed at the current location in the input stream. Appending /N to such commands will prevent the included file’s contents from being echoed at the start of the output file.
• Comments begin with an exclamation point (!), which may appear anywhere on a line. Separate comment lines may appear anywhere within the input file.