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Preparing and Submitting Gaussian Calculations

After constructing and editing a molecular structure in GaussView, the next step is to prepare a Gaussian calculation. GaussView provides a graphical interface that automatically generates the required Gaussian input file, allowing users to configure computational methods and calculation types without manually writing Gaussian keywords.

Almost every Gaussian calculation follows the same general workflow:

Build Molecule
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      ▼
Calculation Setup
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      ▼
Select Job Type
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      ▼
Choose Method
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      ▼
Choose Basis Set
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      ▼
Specify Charge & Multiplicity
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Submit Gaussian Job
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Analyze Results

The only difference between various calculations is the Job Type selected during the setup.

Opening the Gaussian Calculation Setup Window

Once the molecular structure has been completed,

  1. Click Calculate from the menu bar.
  2. Select Gaussian Calculation Setup.

This opens the Gaussian Calculation Setup window, where all computational parameters are specified before submitting the calculation.

Gaussian Calculation Setup Window

Figure. Type of Jobs available.

The setup window is divided into several tabs, each controlling a different aspect of the calculation.

Tab Purpose
Job Type Select the type of calculation to perform
Method Choose the electronic structure method and basis set
Title Provide a descriptive title for the calculation
Link 0 Specify memory, processors, checkpoint file, and other execution options
General General Gaussian settings
Guess Control the initial molecular orbital guess
NBO Natural Bond Orbital analysis
PBC Periodic boundary condition calculations
Solvation Implicit solvent models
Add. Imp. Additional implementation-specific keywords

For most routine calculations, only the Job Type, Method, and Link 0 tabs need to be modified.

Selecting the Job Type

The Job Type determines what Gaussian will calculate.

The available job types include

Job Type Purpose
Energy Compute the electronic energy of the current molecular geometry.
Optimization Optimize the molecular geometry to a stationary point.
Frequency Compute vibrational frequencies and thermochemical properties.
Opt + Freq Perform geometry optimization followed immediately by a frequency calculation.
IRC Follow the intrinsic reaction coordinate from a transition state.
Scan Scan one or more internal coordinates to generate a potential energy surface.
Stability Test whether the converged wavefunction corresponds to a stable SCF solution.
NMR Compute magnetic shielding tensors and NMR chemical shifts.

Each of these calculation types will be discussed in detail in subsequent chapters.

In optimization there are several options available..

Each of these has its own routine and significance which can be found here.

Choosing the Computational Method

The Method tab allows the user to specify

  • Electronic structure method
  • Basis set
  • Spin treatment
  • Dispersion corrections
  • Solvation models (if required)

Examples of commonly used methods include

  • Hartree–Fock (HF)
  • Density Functional Theory (DFT)
  • MP2
  • TD-DFT

Similarly, common basis sets include

  • STO-3G
  • 3-21G
  • 6-31G
  • 6-31G(d)
  • 6-311++G(d,p)
  • cc-pVDZ
  • cc-pVTZ

The choice of computational method depends on the desired balance between computational cost and accuracy. Method selection will be discussed in later tutorials.

Charge and Multiplicity

Every Gaussian calculation requires specifying the molecular charge and spin multiplicity.

Examples include

System Charge Multiplicity
Neutral closed-shell molecule 0 1
Cation +1 1 or 2
Anion –1 1 or 2
Radical 0 2

For most molecules encountered in introductory calculations, the default values are

Charge = 0
Multiplicity = 1

Additional Keywords

The Additional Keywords field allows experienced users to manually add Gaussian route keywords that are not directly available through the graphical interface.

Examples include

Pop=Full
Density=Current
SCF=Tight
Integral=UltraFine

For beginners, this field can usually be left empty.

Link 0 Commands

The Link 0 tab controls how Gaussian utilizes the available hardware.

Typical options include

  • Number of processors
  • Memory allocation
  • Checkpoint file
  • Scratch file options

A typical Link 0 section generated by GaussView may appear as

%chk=water.chk
%mem=8GB
%nprocshared=8

These settings influence the computational resources available to Gaussian but do not affect the underlying quantum chemical method.

Submitting the Calculation

Once all settings have been specified,

  1. Review the selected calculation parameters.
  2. Click Submit.
  3. Choose the directory where the calculation should be saved.
  4. Enter an appropriate filename.
  5. Start the Gaussian calculation.

GaussView automatically generates the Gaussian input file (.gjf or .com) and launches the Gaussian executable.

Monitoring the Calculation

During execution, Gaussian writes its progress to an output file.

Depending on the selected calculation, this process may take from a few seconds to several hours.

Upon completion, the output file can be opened directly in GaussView for visualization and analysis.

Next Steps

Now that you know how to prepare and submit a Gaussian job, the following chapters explain each calculation type individually.

The remaining tutorials will cover

  • Single Point Energy
  • Geometry Optimization
  • Frequency Calculation
  • Optimization + Frequency
  • Intrinsic Reaction Coordinate (IRC)
  • Coordinate Scan
  • NMR Calculations
  • Stability Analysis

Each chapter will explain the theoretical background, required inputs, interpretation of the output, and practical examples.