Gaussian Input

A. Entering Information in the Gaussian Job Entry Window. The picture at the right shows the Job Entry window of Gaussian 94W for Windows. You must supply information in the Route , Charge & Multiplicity, and Molecule Specification sections. In the Route Section you enter several keywords for the MO procedure, basis set, and type of calculation. The Route Section should begin with #T and end with Test. There are no spaces between letters in the keyword set specifying the theoretical method and basis set. There are spaces before the types of calculations. The Charge and Multiplicity section specifies the charge of the molecule and its spin multiplicity.		The Title Section is not required, but is useful for giving a title to the output of the calculation. The Molecule Specification section specifies the coordinates of the atoms in the molecule.
Two examples of messages for a calculation on formaldehyde are shown below. Note that there are no spaces in part of the Route section line.

The Route Section

The keywords in the Route Section tell the theoretical method, the basis set, and the properties to be calculated. See the separate Web page on Computational Chemistry for a more detailed discussion of theoretical methods and basis sets.

Theoretical Method Keywords

We usually will use one of the following theoretical methods.

• RHF: Restricted Hartree-Fock. Useful for initial predictions of the properties of closed shell systems (all electrons paired).
• UHF: Unrestricted Hartree-Fock. Similar to RHF except that it often is used when the system contains unpaired electrons.
• MP2: A RHF calculation followed by a second calculation that compensates for electron correlation.
• UMP2: An unrestricted MP2 calculation often used when the system contains unpaired electrons.

Basis Set Keywords

The basis set is the set of functions that are combined to represent the molecular orbitals. Usually the quality of a basis set depends on its size - the bigger the better. The 6-31G(d) set is a popular basis set for medium to large systems.

Calculation Keywords

The following keywords are used for various types of Gaussian calculations.

• #T     Specifies terse output (only the essential results). Always used.
• Test     Prevents Gaussian from archiving the results. Always used.
• Opt     Used to optimize the geometry of the molecule.
• Freq     Used when you want to calculate the vibrational frequencies or the enthalpy of a molecule.
• Pop=Reg     Used when you want to display the five highest occupied and five lowest unoccupied molecular orbitals.
• Pop=Full     Used when you want to display all the molecular orbitals.

The Charge and Multiplicity Section

Generally molecules will be uncharged. The spin multiplicity depends on the number of unpaired electrons in the molecule.

The Molecule Specification Section

In this section you specify the coordinates of the atoms in the molecule.

Large molecules: The easiest way of specifying the coordinates is to do a preliminary structure of the molecule with a semiempirical molecular orbital program such as CAChe or HyperChem. The structure can be saved as a Brookhaven Protein Data Bank (.pdb) file, which can be imported into Gaussian.

Small molecules: For small molecules, start with an approximate structure of the molecule, either from the literature, from a semiempirical MO program, or from a VSEPR prediction. Calculate the Cartesian coordinates of the atoms from this structure and enter them directly into the Molecular Specification section.

Example

Consider the formaldehyde molecule with bond lengths and bond angles shown on the molecular model at the right. We want to choose Cartesian coordinates for the four atoms in the molecule for input into Gaussian. Orient the flat molecule in the x-y plane with the carbon atom at the origin. The x- and y-positions of the two hydrogen atoms can be calculated using trigonometry: x = 1.09 sin(58.25) = 0.926 y = 1.09 cos(58.25) = 0.573 The Cartesian coordinates of the four atoms are shown below.   x y z C 0.0 0.0 0.0 O 0.0 1.20 0.0 H 0.93 -0.57 0.0 H -0.93 -0.57 0.0

The PCOL community acknowledges that partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE #9950809. Additional support was provided by the Camille and Henry Dreyfus Foundation. PCOL faculty also acknowledge the National Science Teachers Association which awarded the PCOL Faculty Consortium the 1998 Gustav Ohaus Award for Innovation in College Science Teaching.

This site created by David Whisnant (whisnantdm@wofford.edu).
This page was last updated on March 9, 2005
llever@uscupstate.edu