Understanding the Gaussian Input File¶
Although GaussView automatically generates Gaussian input files, understanding their structure is essential for creating, modifying, and troubleshooting calculations. Every Gaussian input file follows a well-defined format, where each section provides specific instructions to the Gaussian program.
The input file used in this tutorial is reproduced below.
%nprocshared=1
%mem=2GB
%chk=formicacid-opt-energy.chk
# b3lyp/6-31g geom=connectivity
Title Card Required
0 1
C -0.11131000 1.89246900 -0.00024600
H -0.16211200 2.98106100 -0.00117300
O 1.14244500 1.44697500 0.00021700
H 1.22534700 0.42975100 0.00008700
O -1.14244500 1.18616900 0.00010300
C 0.11131000 -1.89246900 -0.00024600
H 0.16211200 -2.98106100 -0.00117300
O -1.14244500 -1.44697500 0.00021700
O 1.14244500 -1.18616900 0.00010300
H -1.22534700 -0.42975100 0.00008700
1 2 1.0 3 1.5 5 2.0
2
3
4
5
6 7 1.0 8 1.5 9 2.0
7
8
9
10
Overall Structure¶
A Gaussian input file is organized into six logical sections.
Link 0 Commands
│
▼
Route Section
│
▼
Title Section
│
▼
Charge and Multiplicity
│
▼
Cartesian Coordinates
│
▼
Connectivity Section
Each section has a specific purpose and must appear in the correct order.
1. Link 0 Commands¶
The Link 0 commands appear at the beginning of every Gaussian input file. These commands define the computational resources that Gaussian will use during the calculation.
%nprocshared¶
Specifies the number of CPU cores available to Gaussian.
For example,
instructs Gaussian to use eight shared-memory processors.
%mem¶
Defines the maximum amount of RAM available for the calculation.
Examples include
Larger and more complex calculations generally require more memory.
%chk¶
Creates a checkpoint file, which stores important information generated during the calculation, including
- Molecular orbitals
- Wavefunction
- Geometry
- Density matrix
- Restart information
Checkpoint files are frequently reused in subsequent Gaussian calculations.
2. Route Section¶
The Route Section is the most important line in the Gaussian input file. It specifies
- the type of calculation,
- the electronic structure method,
- the basis set,
- and any additional computational keywords.
For this example,
| Keyword | Meaning |
|---|---|
# |
Indicates the beginning of the Route Section |
B3LYP |
Density Functional Theory method |
6-31G |
Basis set |
geom=connectivity |
Read molecular connectivity from the input file |
Since no keyword such as Opt or Freq is present, Gaussian performs a Single Point Energy calculation.
3. Title Section¶
The title section is a user-defined description of the calculation.
For example,
This text is printed in the Gaussian output file and helps identify the calculation. It has no effect on the computational results.
4. Charge and Multiplicity¶
The first number specifies the total molecular charge, while the second specifies the spin multiplicity.
For the present example,
| Quantity | Value |
|---|---|
| Charge | 0 |
| Multiplicity | 1 |
A multiplicity of 1 corresponds to a closed-shell singlet molecule with all electrons paired.
5. Cartesian Coordinates¶
The Cartesian coordinate section specifies the three-dimensional geometry of the molecule.
Each line contains
The coordinates are expressed in Ångström (Å) unless specified otherwise.
For a Single Point Energy calculation, these coordinates remain fixed throughout the calculation.
6. Connectivity Section¶
The connectivity section specifies which atoms are chemically bonded.
Each line contains
For example,
indicates
- Atom 1 is bonded to Atom 2
- Bond order = 1 (single bond)
Similarly,
represents a bond order of 1.5, commonly used to describe delocalized or aromatic bonding.
This section is required only when the keyword
appears in the Route Section.
Summary¶
The Gaussian input file consists of six logical sections:
| Section | Purpose |
|---|---|
| Link 0 Commands | Define computational resources |
| Route Section | Specify the calculation type, method, and basis set |
| Title Section | Provide a descriptive title |
| Charge & Multiplicity | Define the electronic state of the molecule |
| Cartesian Coordinates | Specify the molecular geometry |
| Connectivity | Define chemical bonding between atoms |
Although GaussView generates these sections automatically, understanding their purpose allows users to modify calculations manually, troubleshoot errors, and prepare more advanced Gaussian input files.