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.

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.

For convenience, the following table all possible sections that might appear within a Gaussian 09 input file, along with the keywords associated with each one.

Gaussian 09 Input Section Ordering

SectionKeywordsFinal blank line?
Link 0 commands% commandsno
Route Section (# lines)allyes
Extra OverlaysExtraOverlaysyes
Title sectionall except Geom=AllCheckyes
Molecule specificationall except Geom=AllCheckyes
Connectivity specificationsGeom=Connect or ModConnectyes
Alterations to frozen atomsGeom=ReadFreezeyes
Modifications to coordinatesOpt=ModRedundantyes
2nd title and molecule specificationOpt=QST2 or QST3yes for both
Connectivity specs. for 2nd set of coordinatesGeom=Connect or ModConnect and Opt=QST2 or QST3yes
2nd Alterations to frozen atomsGeom=ReadFreezeyes
Modifications to 2nd set of coordinatesOpt=QST2 or QST3yes
3rd title and initial TS structureOpt=QST3yes for both
Connectivity specs. for 3rd set of coordinatesGeom=Connect or ModConnect Opt=(ModRedun, QST3)yes
3rd Alterations to frozen atomsGeom=ReadFreezeyes
Modifications to 3rd set of coordinatesOpt=(ModRedun, QST3)yes
PDB secondary structure informationautomatic if residue info in molecule specificationyes
Atomic massesReadIsotopes optionyes
Molecular Mechanics parametersHardFirst, SoftFirst, SoftOnly, Modifyyes
Frequency of interestCPHF=RdFreqyes
Background charge distributionChargeyes
BOMD/ADMP input (1 or more sections)ADMP and BOMD required input and ReadVelocity, ReadMWVelocity optionsyes
PCM inputSCRF=(ExternalIteration,Read)yes
Coordinates for IRC tableIRC=Reportyes
Harmonic constraintsGeom=ReadHarmonicyes
Semi-empirical parameters (Gaussian format)Input option, AM1=Bothyes
Semi-empirical parameters (MOPAC format)MOPAC, Both optionsyes
Basis set specificationGen, GenECP, ExtraBasisyes
Basis set alterationsMassageyes
Finite field coefficientsField=Readyes
ECP specificationPseudo=Cards, GenECPyes
Density fitting basis set specificationExtraDensityBasisyes
PCM solvation model inputSCRF=Readyes
DFTB parametersDFTByes
Source for initial guessGuess=Inputyes
Symmetry types to combineGuess=LowSymmno
Orbital specifications (separate α & β)Guess=Cardsyes
Orbital alterations (separate α & β)Guess=Alteryes
Orbital reordering (separate α & β)Guess=Permuteyes
# Orbitals/GVB pairGVBno
Weights for CAS state averagingCASSCF=StateAverageno
States of interest for spin orbit couplingCASSCF=SpinOrbitno
Orbital freezing informationReadWindow optionsyes
EPT orbitals to refineEPT=ReadOrbitalsyes
Atoms list for spin-spin coupling constantsNMR=ReadAtomsyes
Alternate atomic radiiPop=ReadRadii or ReadAtRadiiyes
Data for electrostatic propertiesProp=Read or Optyes
NBO inputPop=NBOReadno
Harmonic Normal Mode selectionFreq=SelectNormalModesyes
Hindered Rotor inputFreq=ReadHinderedyes
Anharmonic Normal Mode selectionFreq=SelectAnharmonicNormalModesyes
Normal Modes for FCHTFreq=SelectFCHTNormalModesyes
Input for AnharmonicFreq=ReadAnharmonicyes
Input for FCHTFreq=ReadFCHTyes
Pickett output filenameOutput=Pickettno
PROAIMS output filenameOutput=WFNno

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 09 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 09 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.


Last update: 23 April 2013