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Geometry Optimization Input File

A Geometry Optimization calculation in Gaussian is performed by including the Opt keyword in the route section of the input file. During the calculation, Gaussian repeatedly performs Self-Consistent Field (SCF) calculations, evaluates the forces acting on the atoms, and updates the molecular geometry until the convergence criteria are satisfied.

In this documentation, the geometry optimization of a formic acid dimer is carried out using the B3LYP density functional together with the 6-31G basis set.

Complete Input File

%nprocshared=1
%mem=2GB
%chk=formicacid-opt-only.chk
# opt b3lyp/6-31g geom=connectivity

Geometry Optimization of Formic Acid Dimer

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

Input File Explanation

The Gaussian input file consists of five main sections:

  1. Link 0 Commands
  2. Route Section
  3. Title Section
  4. Molecular Specification
  5. Connectivity Section

Each section has a specific purpose during the calculation.

The first few lines beginning with the % symbol are called Link 0 commands. They control how Gaussian allocates computational resources before the calculation begins.

%nprocshared=1

Specifies the number of processor cores to be used.

  • 1 indicates that the calculation will use a single CPU core.
  • On multicore systems, this value can be increased to reduce computation time.

Example

%nprocshared=8

uses eight processor cores.

%mem=2GB

Specifies the maximum amount of RAM available to Gaussian.

In this calculation,

  • 2 GB of memory is allocated.

For larger molecular systems, increasing the available memory often improves computational efficiency.

%chk=formicacid-opt-only.chk

Creates a checkpoint file.

The checkpoint file stores important information, including

  • molecular orbitals,
  • electron density,
  • optimized geometry,
  • wavefunction,
  • SCF information.

This file can later be reused for

  • Frequency calculations,
  • NMR calculations,
  • IRC calculations,
  • Excited-state calculations,
  • Restarting interrupted jobs.

2. Route Section

The route section defines the type of calculation and the theoretical method.

# opt b3lyp/6-31g geom=connectivity

Each keyword has a specific meaning.

Opt

Opt

Requests a Geometry Optimization.

Gaussian will

  • calculate the electronic energy,
  • evaluate the nuclear forces,
  • move the atoms,
  • repeat the procedure

until an optimized molecular structure is obtained.

Without this keyword, Gaussian performs only a Single Point Energy calculation.

B3LYP

B3LYP

Specifies the Density Functional Theory (DFT) method.

B3LYP is a hybrid exchange-correlation functional that combines

  • Hartree–Fock exchange,
  • Becke exchange,
  • Lee–Yang–Parr correlation.

It offers an excellent balance between computational cost and accuracy for many molecular systems.

6-31G

6-31G

Specifies the basis set used to describe the molecular orbitals.

This split-valence Gaussian basis set provides a good compromise between

  • computational efficiency,
  • numerical stability,
  • chemical accuracy.

Geom=Connectivity

Geom=Connectivity

Instructs Gaussian to read both

  • Cartesian coordinates
  • bonding connectivity

from the input file.

This preserves the molecular bonding information generated by GaussView and is especially useful for larger molecules.

3. Title Section

Geometry Optimization of Formic Acid Dimer

The title section is a user-defined description of the calculation.

It has no effect on the calculation itself but appears in the output file and helps identify the job.

4. Molecular Specification

0 1

These two numbers specify the molecular charge and spin multiplicity.

  • 0 → Neutral molecule
  • 1 → Singlet electronic state

For radicals or ions, these values must be modified accordingly.

Cartesian Coordinates

The Cartesian coordinates define the initial molecular geometry.

C   1.23397847   0.39209922  -0.08248242

Each line contains

  • atom symbol,
  • X coordinate,
  • Y coordinate,
  • Z coordinate.

These coordinates provide the starting structure for the optimization.

During the optimization, Gaussian updates these coordinates repeatedly until the optimized geometry is reached.

5. Connectivity Section

The final part of the input specifies the molecular connectivity.

For example,

1 2 1.0 3 1.0 5 2.0

indicates that atom 1 is connected to

  • atom 2 by a single bond,
  • atom 3 by a single bond,
  • atom 5 by a double bond.

Although Gaussian can often determine bonding automatically, including the connectivity explicitly ensures that the molecular structure generated in GaussView is interpreted correctly.

Summary

This input file performs a Geometry Optimization of the formic acid dimer using the B3LYP hybrid density functional and the 6-31G basis set. The Link 0 commands allocate computational resources and create a checkpoint file, the route section defines the theoretical method and optimization procedure, the molecular specification provides the initial geometry, and the connectivity section preserves the bonding information. During the calculation, Gaussian repeatedly updates the atomic coordinates until the optimized molecular structure is obtained.

Next Section

The next chapter explains how to interpret the Geometry Optimization output file, including the optimization cycles, energy changes, convergence criteria, and the final optimized geometry.