Frequency Calculation Input¶
A Gaussian Frequency Calculation requires an input file containing the computational resources, route section, molecular charge and multiplicity, atomic coordinates, and molecular connectivity.
The input file used throughout this documentation performs a Density Functional Theory (DFT) frequency calculation on the formic acid dimer using the B3LYP functional and the 6-31G basis set.
Complete Input File¶
%nprocshared=1
%mem=2GB
%chk=formicacid-freq.chk
# freq 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
Structure of a Gaussian Input File¶
A Gaussian input file is divided into several logical sections.
Link 0 Commands
│
▼
Route Section
│
▼
Title Section
│
▼
Charge and Multiplicity
│
▼
Atomic Coordinates
│
▼
Connectivity (Optional)
Each section has a specific purpose during the calculation.
Link 0 Commands¶
The first three lines allocate computational resources.
%nprocshared=1¶
Specifies the number of processor cores used by Gaussian.
For larger calculations, this value may be increased to match the available CPU cores.
Examples
Increasing the number of processors generally reduces the computational time.
%mem=2GB¶
Specifies the amount of RAM available to Gaussian.
The memory should be chosen according to the available system resources and the size of the molecular system.
Examples
Larger basis sets and larger molecules usually require more memory.
%chk=formicacid-freq.chk¶
The checkpoint file stores important information generated during the calculation, including
- molecular geometry,
- molecular orbitals,
- wavefunction,
- Hessian matrix,
- vibrational information.
The checkpoint file can later be reused for additional calculations such as
- IR spectra,
- Raman spectra,
- NMR calculations,
- Single Point Energy calculations,
- excited-state calculations.
Route Section¶
The route section specifies the type of calculation and the quantum chemical method.
This line tells Gaussian exactly what calculation should be performed.
freq¶
Requests a vibrational frequency calculation.
Gaussian computes
- the Hessian matrix,
- vibrational frequencies,
- normal modes,
- infrared intensities,
- thermodynamic properties,
- Zero-Point Energy.
Unlike Geometry Optimization, the molecular geometry is not modified during a Frequency Calculation.
b3lyp¶
Specifies the electronic structure method.
B3LYP is a hybrid Density Functional Theory (DFT) functional that combines
- Hartree–Fock exchange,
- Becke exchange,
- Lee–Yang–Parr correlation.
It offers a good balance between computational efficiency and accuracy and is widely used for molecular vibrational studies.
6-31G¶
Defines the basis set used to represent the molecular orbitals.
The basis set determines how accurately the electronic wavefunction is described.
Larger basis sets generally produce more accurate frequencies but require greater computational resources.
geom=connectivity¶
Instructs Gaussian to read the molecular connectivity supplied after the Cartesian coordinates.
Connectivity information
- defines chemical bonds,
- preserves the intended molecular structure,
- assists in constructing internal coordinates.
This keyword is especially useful for molecules generated using GaussView.
Title Section¶
This line provides a descriptive title for the calculation.
It has no effect on the numerical results but helps identify calculations.
For example,
Charge and Multiplicity¶
The first number specifies the molecular charge.
The second specifies the spin multiplicity.
For this calculation,
| Quantity | Value |
|---|---|
| Charge | 0 |
| Multiplicity | 1 |
This corresponds to a neutral molecule in the singlet electronic state.
Molecular Geometry¶
The Cartesian coordinates define the positions of all atoms.
Each line contains
- atomic symbol,
- x-coordinate,
- y-coordinate,
- z-coordinate.
The coordinates used here correspond to the molecular structure that will be analyzed for its vibrational properties.
Important: A Frequency Calculation should normally be performed on an optimized geometry. Running a frequency calculation on a non-optimized structure may produce imaginary frequencies that arise from the geometry not being at a stationary point, rather than indicating a true transition state.
Connectivity Section¶
The final section specifies the bonding pattern.
The numbers indicate
- atom numbers,
- bonded atoms,
- bond orders.
For example,
indicates that atom 1 is connected to atom 2 by a single bond.
Similarly,
represents a bond order of 1.5, commonly used for delocalized or resonance structures.
Although optional in many Gaussian calculations, connectivity information is recommended when the input file is generated from GaussView.
Summary¶
A Gaussian Frequency Calculation input file consists of Link 0 commands, the route section, a title, molecular charge and multiplicity, Cartesian coordinates, and optional connectivity information. The freq keyword requests the calculation of vibrational frequencies and thermodynamic properties, while the selected electronic structure method and basis set determine the level of theory. Once the input file is prepared, Gaussian computes the Hessian matrix, from which vibrational frequencies, normal modes, infrared intensities, and thermodynamic quantities are obtained.
Next Section¶
The next chapter explains the Gaussian Frequency Output, including the interpretation of vibrational frequencies, imaginary frequencies, normal modes, infrared intensities, Zero-Point Energy, and the thermodynamic quantities reported in the output file.