The Gaussian Computational Chemistry Program

Adapted from http://www.chm.bris.ac.uk/pt/harvey/msci_pract/tech_g98.html

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Gaussian is a complicated Fortran program which can perform a variety of computational tasks such as computing energies, optimizing geometries, computing vibrational frequencies, etc., and with a variety of computational methods, including simple Molecular Mechanics (force-field) approaches, Semi-empirical wavefunction-type methods, Hartree-Fock Self-Consistent Field methods, so-called "post-Hartree-Fock" or "correlated" methods, and Density Functional Theory approaches. Here you will learn how to:

• Generate Gaussian input files.
• Issue Commands to Gaussian.
• Specify the Molecular Geometry.
• Run a Gaussian Job.
• Read Gaussian output files.

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Gaussian Input Files

Gaussian input is always in the form of a unformatted text or ASCII file, usually called 'jobname'.com. For historical reasons (Gaussian has been around for more than 25 years, and new versions try to be partly compatible with older ones, so the input format has not changed much), the order of the different items in the file has to be followed very rigorously, because otherwise Gaussian does not understand the input (typically, it tries to find piece of information 'A' where you have written 'B', and that makes no sense). When this happens, the job stops, and you get a rude & incomprehensible message in the Gaussian Output (or logfile). Whenever this happens, consult the following section!!:
There are two types of lines of input in Gaussian:

• Single-line Input. This is on a single, self-contained line. E.g. the 'Charge and Spin' data below.
• Multiple-line Input. This is on any number of lines. For Gaussian to know where this group stops, it needs to be followed by ONE blank line. Example: the geometry specification.

Here is a sample Gaussian Input File:

The sections indicated in this file are the most common input groups. In the order in which they should appear in Gaussian, these are:

'Link 0' commands. This is the way Gaussian designates general commands concerning the way in which the program should run, what it does with files, etc. For most jobs, you will not need any Link 0 command. The line shown here, which starts with the % sign, tells Gaussian to save to your current directory the temporary 'Checkfile', and to call it ethene.chk. If you do not do this, the checkfile is kept in a temporary, 'work', directory, and deleted when the job ends. Link 0 commands are Single-line Input.

The Command Section. This is the most important part of the input: It tells Gaussian what to do ! Although it is only one line long here, it is Multiple-line Input and needs to be followed by one blank line. As with other Gaussian input, this section is case-insensitive. The first line of the Command Section needs to start with the '#' sign. Here, the program is being instructed to perform a geometry optimisation (opt) of the specified molecule, at the Hartree-Fock level of theory, and using the 3-21G standard basis set on all atoms, which it will read from its basis set library.

The Job Title. This is free for you to choose. It can be as brief or as long as you like, and may span several lines. It is multiple-line input, so it needs to be finished by a blank line. As a more verbose alternative to the dull job title I have put here, the following would be fine, too:

This file is the first Gaussian Input File I look at.
Running it will cause Gaussian to perform
an ab initio Hartree-Fock Geometry
Optimisation of the ethene molecule!!!!

The Molecular Charge and Spin setting. This begins your description of the molecule you wish Gaussian to calculate. You need to specify the overall molecular charge and the spin multiplicity (as two integers). Here, ethene is a neutral species (charge=0), and it is a spin-Singlet, with S=0 and the multiplicity, 2*S + 1, equal to 1.

The Geometry Specification. This is multiple-line input, so it needs to be followed by a blank line. Because it is often the last part of a Gaussian input file, you may forget to add a blank line, especially as with pico, you cannot see whether a blank line is there or not ! The format can be Cartesian, as here, or in Z-matrix format (see below).

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To continue reading click on the following link:

http://inka.mssm.edu/~mezei/molmod_course/gauss_tut.html#g98_input