Gaussian 03 Input Overview

Gaussian 03 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).

• 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 03 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.

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

%Chk=heavy                                   Link 0 section 
#HF/6-31G(d) Opt=ModRedundant                Route section  

Opt job                                      Title section 

 0    1                                       Molecule Specification section 
atomic coordinates …        

3 8                                          Add a bond and an angle to the internal 
2 1 3                                         coordinates used during the geom. opt.  

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.

Link 0 commands were introduced in the last chapter and are discussed individually in the penultimate section of this chapter. The remaining input sections are discussed in the subsequent subsections of this introductory section. For convenience, the table below lists all possible sections that might appear within a Gaussian 03 input file, along with the keywords associated with each one.

Input Syntax

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 03 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 03 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.

Gaussian 03 Input Section Ordering

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