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

Section	Keywords	Final blank line?
Link 0 commands	% commands	no
Route Section (# lines)	all	yes
Extra Overlays	ExtraOverlays	yes
Title section	all except Geom=AllCheck	yes
Molecule specification	all except Geom=AllCheck	yes
Connectivity specifications	Geom=Connect or ModConnect	yes
Alterations to frozen atoms	Geom=ReadFreeze	yes
Modifications to coordinates	Opt=ModRedundant	yes
2nd title and molecule specification	Opt=QST2 or QST3	yes for both
Connectivity specs. for 2nd set of coordinates	Geom=Connect or ModConnect and Opt=QST2 or QST3	yes
2nd Alterations to frozen atoms	Geom=ReadFreeze	yes
Modifications to 2nd set of coordinates	Opt=QST2 or QST3	yes
3rd title and initial TS structure	Opt=QST3	yes for both
Connectivity specs. for 3rd set of coordinates	Geom=Connect or ModConnect Opt=(ModRedun, QST3)	yes
3rd Alterations to frozen atoms	Geom=ReadFreeze	yes
Modifications to 3rd set of coordinates	Opt=(ModRedun, QST3)	yes
PDB secondary structure information	automatic if residue info in molecule specification	yes
Atomic masses	ReadIsotopes option	yes
Molecular Mechanics parameters	HardFirst, SoftFirst, SoftOnly, Modify	yes
Frequency of interest	CPHF=RdFreq	yes
Background charge distribution	Charge	yes
BOMD/ADMP input (1 or more sections)	ADMP and BOMD required input and ReadVelocity, ReadMWVelocity options	yes
PCM input	SCRF=(ExternalIteration,Read)	yes
Coordinates for IRC table	IRC=Report	yes
Harmonic constraints	Geom=ReadHarmonic	yes
Semi-empirical parameters (Gaussian format)	Input option, AM1=Both	yes
Semi-empirical parameters (MOPAC format)	MOPAC, Both options	yes
Basis set specification	Gen, GenECP, ExtraBasis	yes
Basis set alterations	Massage	yes
Finite field coefficients	Field=Read	yes
ECP specification	Pseudo=Cards, GenECP	yes
Density fitting basis set specification	ExtraDensityBasis	yes
PCM solvation model input	SCRF=Read	yes
DFTB parameters	DFTB	yes
Source for initial guess	Guess=Input	yes
Symmetry types to combine	Guess=LowSymm	no
Orbital specifications (separate α & β)	Guess=Cards	yes
Orbital alterations (separate α & β)	Guess=Alter	yes
Orbital reordering (separate α & β)	Guess=Permute	yes
# Orbitals/GVB pair	GVB	no
Weights for CAS state averaging	CASSCF=StateAverage	no
States of interest for spin orbit coupling	CASSCF=SpinOrbit	no
Orbital freezing information	ReadWindow options	yes
EPT orbitals to refine	EPT=ReadOrbitals	yes
Atoms list for spin-spin coupling constants	NMR=ReadAtoms	yes
Alternate atomic radii	Pop=ReadRadii or ReadAtRadii	yes
Data for electrostatic properties	Prop=Read or Opt	yes
NBO input	Pop=NBORead	no
Harmonic Normal Mode selection	Freq=SelectNormalModes	yes
Hindered Rotor input	Freq=ReadHindered	yes
Anharmonic Normal Mode selection	Freq=SelectAnharmonicNormalModes	yes
Normal Modes for FCHT	Freq=SelectFCHTNormalModes	yes
Input for Anharmonic	Freq=ReadAnharmonic	yes
Input for FCHT	Freq=ReadFCHT	yes
Pickett output filename	Output=Pickett	no
PROAIMS output filename	Output=WFN	no

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 updated on: 9 Jun 2009