Gaussian Input File¶
An Optimization + Frequency calculation combines Geometry Optimization and Vibrational Frequency Analysis into a single Gaussian job. Instead of performing these calculations separately, Gaussian first optimizes the molecular geometry and then immediately performs a frequency calculation using the optimized structure.
The input file used in this tutorial is shown below.
Complete Input File¶
%nprocshared=1
%mem=2GB
%chk=formicacid-opt+freq.chk
# opt freq b3lyp/6-31g geom=connectivity
Title Card Required
0 1
C 1.23397847 0.39209922 -0.08248242
H 0.57143844 1.10635833 -0.52494669
O 2.35792280 0.82941585 0.68448021
H 3.06522065 0.18655830 0.58859319
O 1.03139137 -0.84041264 -0.23351348
C 4.66402878 -2.75634677 0.80613602
H 5.43907457 -3.49119962 0.74136491
O 3.31406011 -3.15229051 1.05821047
O 4.93252753 -1.53589978 0.66000260
H 2.72448326 -2.53098133 0.62334906
1 2 1.0 3 1.0 5 2.0
2
3 4 1.0
4
5
6 7 1.0 8 1.0 9 2.0
7
8 10 1.0
9
10
Structure of the Input File¶
The Gaussian input file consists of the following sections:
Link-0 Commands
│
▼
Route Section
│
▼
Title Section
│
▼
Charge & Multiplicity
│
▼
Cartesian Coordinates
│
▼
Connectivity Information
Each section has a specific purpose and must appear in the correct order.
Link-0 Commands¶
The first three lines allocate the computational resources required for the calculation.
%nprocshared=1¶
Specifies the number of CPU cores that Gaussian should use.
Increase this value when running on multicore processors to reduce the calculation time.
%mem=2GB¶
Defines the maximum memory available to Gaussian.
Larger molecular systems generally require more memory for efficient calculations.
%chk=formicacid-opt+freq.chk¶
Specifies the checkpoint file.
The checkpoint file stores
- optimized molecular geometry,
- molecular orbitals,
- wavefunction,
- basis set information,
- vibrational data.
It can be reused in later Gaussian calculations without repeating the optimization.
Route Section¶
The Route Section defines the type of calculation to be performed.
This is the most important line in the entire input file.
Each keyword controls a different aspect of the calculation.
opt¶
Requests a Geometry Optimization.
Gaussian iteratively adjusts the atomic coordinates until the molecular energy reaches a stationary point.
During this step,
- bond lengths,
- bond angles,
- dihedral angles
are optimized to obtain the equilibrium molecular geometry.
freq¶
Requests a Vibrational Frequency Calculation.
Immediately after the optimization converges,
Gaussian
- constructs the Hessian matrix,
- calculates the normal modes,
- determines the vibrational frequencies,
- predicts IR intensities,
- computes thermodynamic properties.
No additional input is required because the optimized geometry from the previous step is used automatically.
b3lyp/6-31g¶
Specifies the electronic structure method and basis set.
B3LYP¶
A hybrid Density Functional Theory (DFT) functional widely used for
- molecular geometries,
- vibrational frequencies,
- reaction energetics,
- thermochemistry.
6-31G¶
A split-valence basis set suitable for small and medium-sized molecules.
Together,
B3LYP/6-31G provides a good balance between computational cost and accuracy for introductory calculations.
geom=connectivity¶
Instructs Gaussian to read the molecular connectivity supplied after the Cartesian coordinates.
The connectivity table specifies
- bonded atoms,
- bond orders,
- molecular topology.
This information is especially useful when molecules contain
- aromatic systems,
- unusual bond orders,
- multiple fragments.
Title Section¶
The title section is simply a user-defined description of the calculation.
For example,
Although any text may be used,
a descriptive title makes it easier to identify calculations later.
Charge and Multiplicity¶
These two numbers specify
- molecular charge,
- spin multiplicity.
For this calculation
- Charge = 0
- Multiplicity = 1
indicating a neutral closed-shell singlet molecule.
Cartesian Coordinates¶
The Cartesian coordinates specify the initial molecular geometry.
Each line contains
- element symbol,
- X coordinate,
- Y coordinate,
- Z coordinate.
These coordinates serve as the starting point for the geometry optimization.
Gaussian modifies these coordinates iteratively until the optimized structure is obtained.
Connectivity Table¶
The final section defines the molecular bonding.
Example
This indicates
- atom 1 is connected to atom 2 by a single bond,
- atom 1 is connected to atom 3 by a single bond,
- atom 1 is connected to atom 5 by a double bond.
The connectivity table ensures that Gaussian correctly interprets the molecular structure.
Workflow of the Calculation¶
The calculation proceeds automatically as
Read Molecular Structure
│
▼
Geometry Optimization
│
▼
Optimized Molecular Geometry
│
▼
Frequency Calculation
│
▼
Normal Modes
│
▼
IR Intensities
│
▼
Thermodynamic Properties
Unlike separate calculations, no user interaction is required between the optimization and frequency steps.
Why Use an Optimization + Frequency Job?¶
Combining both calculations into a single job offers several advantages.
- The frequency calculation automatically uses the optimized geometry.
- Only one input file needs to be prepared.
- Eliminates the possibility of performing a frequency calculation on an unoptimized structure.
- Saves computational setup time.
- Produces structural, vibrational, and thermodynamic information in a single Gaussian output file.
For routine molecular studies, this combined workflow is the recommended approach.
Summary¶
The input file for an Optimization + Frequency calculation combines the keywords required for both Geometry Optimization and Vibrational Frequency Analysis. Gaussian first optimizes the molecular structure using the selected electronic structure method and basis set. Once convergence is achieved, it immediately performs a frequency calculation on the optimized geometry, generating vibrational frequencies, infrared intensities, and thermodynamic properties. By combining both calculations into a single input file, users obtain a complete characterization of the molecular structure efficiently and with minimal manual intervention.