Skip to content

Understanding the Gaussian Output

After the calculation is submitted, Gaussian writes all computational information to an output file (.log or .out). This file contains details of the input structure, computational settings, Self-Consistent Field (SCF) iterations, calculated energies, molecular properties, and the final status of the calculation.

Although a Gaussian output file may contain hundreds or even thousands of lines, only a few sections are routinely examined for a Single Point Energy calculation.

This chapter explains the most important sections of the output using the formic acid dimer example.

General Structure of the Output File

A typical Single Point Energy output consists of the following sections.

Program Information
        │
        ▼
Input Summary
        │
        ▼
Molecular Geometry
        │
        ▼
Basis Set Information
        │
        ▼
SCF Iterations
        │
        ▼
Final SCF Energy
        │
        ▼
Molecular Properties
        │
        ▼
CPU Timing
        │
        ▼
Normal Termination

Program Information

Near the beginning of the output, Gaussian reports

  • Gaussian version
  • Compilation information
  • Calculation date
  • Number of processors
  • Memory allocation

This section verifies that the calculation has started using the expected computational resources.

Molecular Specification

The output then reproduces the molecular information supplied in the input file, including

  • Atomic coordinates
  • Charge
  • Spin multiplicity
  • Number of atoms
  • Number of electrons

This allows the user to verify that the intended molecular system has been read correctly.

Basis Set Information

Gaussian reports the basis set used for the calculation.

For this example,

Standard basis:
6-31G

Additional information includes

  • Number of basis functions
  • Primitive Gaussian functions
  • Contracted basis functions

These quantities determine the size of the quantum mechanical calculation.

Self-Consistent Field (SCF) Procedure

The largest section of the output corresponds to the SCF iterations.

During each iteration Gaussian

  1. Constructs the Fock (or Kohn–Sham) matrix.
  2. Solves the Roothaan–Hall equations.
  3. Updates the electron density.
  4. Computes the electronic energy.
  5. Tests for convergence.

The energy gradually approaches a stable value until the convergence criteria are satisfied.

SCF Convergence

Once convergence is achieved, Gaussian prints a message similar to

SCF Done:  E(RB3LYP) =  -379.123456789 A.U.

This is the most important line of a Single Point Energy calculation.

It reports

  • the computational method,
  • the final electronic energy,
  • and the energy unit (Hartree).

The electronic energy corresponds to the molecular geometry supplied in the input file.

Because this is a Single Point Energy calculation, the atomic coordinates remain unchanged throughout the calculation.

Molecular Orbital Information

Following SCF convergence, Gaussian prints the orbital energies.

These include

  • Occupied molecular orbitals
  • Virtual molecular orbitals
  • HOMO energy
  • LUMO energy

These orbital energies provide qualitative insight into

  • molecular stability,
  • chemical reactivity,
  • and the HOMO–LUMO energy gap.

Dipole Moment

Gaussian computes the molecular dipole moment using the converged electron density.

The output typically reports

Dipole moment (field-independent basis)

followed by

  • X component
  • Y component
  • Z component
  • Total dipole moment

The dipole moment provides information about the molecular charge distribution and polarity.

Population Analysis

Depending on the selected keywords, Gaussian may also report

  • Mulliken atomic charges
  • Atomic spin densities
  • Natural Population Analysis (NPA)
  • Electrostatic charges

These quantities help describe how electrons are distributed throughout the molecule.

Job Timing

Near the end of the output Gaussian reports

  • CPU time
  • Wall-clock time
  • Elapsed execution time

These statistics are useful for estimating the computational cost of similar calculations.

Normal Termination

The final lines of the output should read

Normal termination of Gaussian

This confirms that

  • the calculation completed successfully,
  • all requested properties were computed,
  • and the reported energies are valid.

If this message is absent, the calculation terminated abnormally and the output should be examined for errors.

Most Important Quantities

For a Single Point Energy calculation, users typically examine only a few key results.

Quantity Why it is Important
SCF Done Energy Final electronic energy of the molecule
Number of SCF Cycles Indicates convergence behavior
HOMO Energy Highest occupied molecular orbital
LUMO Energy Lowest unoccupied molecular orbital
HOMO–LUMO Gap Approximate measure of electronic stability
Dipole Moment Molecular polarity
Normal Termination Confirms successful completion

Summary

Although Gaussian produces extensive output, only a small number of sections are routinely examined for a Single Point Energy calculation. The SCF Done energy is the primary result, while molecular orbital energies, dipole moments, and population analyses provide additional information about the electronic structure. Finally, the presence of Normal termination of Gaussian confirms that the calculation completed successfully and the reported results can be used for further analysis.