Gaussian 09 Input Overview

Gaussian 09 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).
• 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 09 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).
• Optional additional sections: Additional input needed for specific job types (usually blank line terminated).

Many Gaussian 09 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:

# HF/6-31G(d)             Route section

water energy              Title section

0   1                     Molecule specification
O  -0.464   0.177   0.0
H  -0.464   1.137   0.0
H   0.441  -0.143   0.0

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.


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