What Is MOPAC? How to Install and Use Molecular Orbital Calculation Software and Interpret HOMO/LUMO Results

What Is MOPAC?

MOPAC is an abbreviation for
Molecular Orbital PACkage.

It is one of the quantum chemistry calculation programs
used to calculate the electronic states of molecules, solids, and other systems
based on quantum mechanics,
and it mainly performs calculations using semi-empirical molecular orbital methods.

The first version of MOPAC was released in 1983,
and it has been developed for more than 40 years since then.

The current open-source version of MOPAC
directly continues the development of the former commercial version, MOPAC 2016.

It is currently maintained and managed by
the Molecular Sciences Software Institute (MolSSI).

What Kind of Calculations Can MOPAC Perform?

In MOPAC, the electronic state of a molecule can be calculated
by entering the types and positions of its atoms.

Representative calculations include the following.

What Is a Semi-Empirical Molecular Orbital Method?

One of the major characteristics of MOPAC is its use of
semi-empirical molecular orbital methods.

If the electronic state of a molecule is calculated strictly
from quantum mechanics alone,
the amount of computation required becomes extremely large
as the size of the molecule increases.

Semi-empirical molecular orbital methods simplify part of the calculation
while using parameters obtained from experimental values
or high-accuracy calculations.

As a result, they can perform calculations much faster
than methods such as ab initio calculations.

According to the official MOPAC documentation,
semi-empirical calculations can be approximately 1,000 times faster
than ab initio calculations in some cases.
On the other hand, because simplified models and parameters are used,
accuracy and predictive capability may generally be lower
than those of high-accuracy ab initio or DFT calculations.

Representative Calculation Methods Available in MOPAC

MOPAC can use multiple semi-empirical Hamiltonians.

In the current version of MOPAC,
PM7 is used as the standard calculation method
when no method is specifically specified.

PM7 is a general-purpose semi-empirical calculation method
that can be used for a wide range of elements and molecules.

What Is MOPAC Used For?

MOPAC is used in a wide range of fields,
from university education to research.

Learning Molecular Orbitals

Because HOMO, LUMO, molecular orbital energies,
atomic charges, and other quantities can actually be calculated,
MOPAC can be used for studying quantum chemistry and physical chemistry.

Molecular Structure Optimization

When an appropriate initial molecular structure is provided,
MOPAC can change the structure while calculating the forces acting on the atoms
and search for a lower-energy structure.

Calculations of Organic Molecules

It can also be used to compare molecular structures,
electronic states, charge distributions,
HOMO, LUMO, and other properties of organic molecules.

Screening Large Numbers of Molecules

Because semi-empirical calculations are relatively fast,
MOPAC can also be used for high-throughput calculations
involving large numbers of molecules.

Large Molecules Such as Proteins

MOPAC also includes functions such as MOZYME
for handling large molecular systems.

For this reason, it may also be used for calculations
involving proteins with several thousand atoms.

Current Development Status of MOPAC

MOPAC was previously provided as commercial software.

Development of the commercial version ended with MOPAC 2016,
but it was subsequently made open source,
and development and maintenance are still continuing.

The current official source code is publicly available on GitHub.

As of August 2026,
the repository is still being updated,
and MOPAC remains actively maintained software.

Latest Version of MOPAC

As of August 2026,
the latest official version listed on the MOPAC official website is
MOPAC 23.2.5.

Unlike the former MOPAC 2016,
the current MOPAC is provided as an open-source version.

MOPAC Price

The current open-source version of MOPAC
can be used free of charge.

It is released under the Apache License 2.0,
and the source code is also publicly available.

The former commercial version required license authentication,
but the current open-source version does not require
an activation password.

Supported Operating Systems

MOPAC supports the major operating systems used on personal computers.

Official installers and compressed archives
are provided for each operating system.

Installing MOPAC on Windows

On Windows, the Windows installer
can be obtained from the official website.

The installer is provided in
.exe format.

The basic procedure is as follows.

When the official installer is used,
the MOPAC executable file is added to the system PATH.

This allows MOPAC to be run
from Command Prompt or similar interfaces.

Installing MOPAC on macOS

An official installer is also provided for macOS.

The Mac installer is provided in
.dmg format.

macOS Gatekeeper may block the application
the first time it is run.

In that case, it may be possible to run it
by right-clicking the target file in Finder
and selecting “Open.”

Installing MOPAC on Linux

On Linux,
installers in .run format
and archives in .tar.gz format are available.

In addition to the graphical installer,
MOPAC can also be installed from the command line.

On Linux, it may also be available
through the package manager of the distribution.

Installing MOPAC Using Conda

If you use Conda in a Python environment or similar setup,
MOPAC can be installed from conda-forge.

conda install -c conda-forge mopac

This method can be used on Windows, macOS, and Linux
when a Conda environment is available.

Does MOPAC Have a Graphical Interface?

MOPAC itself is basically
a command-line program.

It is somewhat different from ordinary GUI software
in which a molecular structure is created on the screen
and a button is pressed to perform the calculation.

A text-based input file is created
and then read by MOPAC for calculation.

MOPAC itself also does not include
graphical functions for displaying molecules in 3D.

When visualizing molecular structures or molecular orbitals,
MOPAC can be combined with external software
such as Jmol/JSmol or WebMO.

Basic Workflow of MOPAC

The basic workflow of MOPAC is very simple.

What Is a MOPAC Input File?

In MOPAC, molecular structures and calculation conditions
are written in a text file.

The extension
.mop
is generally used.

For example,

water.mop

a file like this can be created.

In addition to .mop,
MOPAC can also handle
.dat and .arc
as input.

Basic Structure of an Input File

A MOPAC input file basically has the following structure.

Keywords
Comment
Comment
Atom 1 coordinates
Atom 2 coordinates
Atom 3 coordinates
...

The first line contains keywords
specifying the calculation method and calculation conditions.

The following two lines are treated as comments,
followed by the atom types and coordinates.

Sample 1: Calculating a Water Molecule

As a first exercise,
calculate a water molecule, H₂O.

Open a text editor
and create a file like the following.

PM7 PRECISE

Water molecule

O  0.000000  0.000000  0.000000
H  0.758602  0.000000  0.504284
H -0.758602  0.000000  0.504284

For example, name the file
water.mop.

This input uses the PM7 method
to optimize the structure of the water molecule.

In MOPAC, PM7 structural optimization is the default operation
even if the calculation method is not specifically specified,
but when learning, explicitly writing PM7
makes the calculation conditions easier to understand.

What Is PRECISE?

PRECISE is a keyword
used to apply stricter convergence criteria than usual.

It is not essential for ordinary educational calculations,
but it can be used when you want to perform structure optimization
with slightly stricter accuracy.

Running MOPAC

If MOPAC is registered in the PATH
and water.mop is located in the current folder,
run it from a terminal or Command Prompt.

mopac water.mop

In MOPAC, it may also be possible
to omit the extension as follows.

mopac water

When the calculation is complete,
output files are created in the same folder
as the input file.

Main Output Files

Multiple files may be created during a MOPAC calculation.

When using MOPAC for the first time,
first check the
.out
file.

What Is Structure Optimization?

The atoms that make up a molecule
are not stable in every possible arrangement.

In MOPAC structure optimization,
the positions of the atoms are gradually changed
from the initial structure provided,
and a lower-energy structure is searched for.

Under the standard settings,
MOPAC performs PM7 structure optimization
on the input structure.

What Is 1SCF?

If you want to calculate only the electronic state
of the input structure without changing the molecular structure,
you can use 1SCF.

PM7 1SCF

Water single point calculation

O  0.000000  0.000000  0.000000
H  0.758602  0.000000  0.504284
H -0.758602  0.000000  0.504284

When 1SCF is specified,
an SCF calculation is performed on the input structure
without structure optimization.

What Are HOMO and LUMO?

HOMO and LUMO are commonly examined
in molecular orbital calculations.

The HOMO is the molecular orbital
with the highest energy among the orbitals occupied by electrons.

The LUMO is the molecular orbital
with the lowest energy among the orbitals not occupied by electrons.

What Can Be Learned from HOMO and LUMO?

HOMO and LUMO are often used
when considering molecular reactivity.

In general,
the HOMO can be associated with the tendency to donate electrons,
while the LUMO can be associated with the tendency to accept electrons.

The energy difference between the HOMO and LUMO
is called the HOMO-LUMO gap.

The HOMO-LUMO gap is used as one indicator
for considering electronic excitation,
molecular reactivity, and other properties.

Checking HOMO and LUMO in MOPAC

In MOPAC,
the VECTORS keyword can be used
when you want detailed molecular orbital information to be output.

PM7 1SCF VECTORS

Water HOMO LUMO calculation

O  0.000000  0.000000  0.000000
H  0.758602  0.000000  0.504284
H -0.758602  0.000000  0.504284

When VECTORS is specified,
the eigenvalues and eigenvectors
of the molecular orbitals are output.

The energies are displayed in eV.

Normally, the boundary between occupied and unoccupied orbitals is identified,
with the highest-energy occupied orbital taken as the HOMO
and the next unoccupied orbital taken as the LUMO.

Specifying the Output Range for VECTORS

MOPAC can also output
only the orbitals around the HOMO and LUMO.

For example,

VECTORS=(3,3)

specifies that three of the highest occupied orbitals
and three of the lowest unoccupied orbitals are output.

If there is no need to examine every orbital,
limiting the output to the region around the HOMO and LUMO
in this way makes the results easier to check.

How to View the Shapes of HOMO and LUMO

MOPAC’s standard output is basically numerical.

Therefore, to examine HOMO and LUMO orbitals
as three-dimensional images,
MOPAC is combined with molecular visualization software.

The official MOPAC guide also introduces
visualization methods using Jmol/JSmol.

By using visualization software,
it is possible to visually examine
which parts of the molecule contain the HOMO
and around which atoms the LUMO is distributed.

Sample 2: Calculating Methane

In the official MOPAC guide,
methane, CH₄,
is also used as a basic input example.

PM7

Methane

C  0.00  0.00  0.00
H  0.63  0.63  0.63
H  0.63 -0.63 -0.63
H -0.63  0.63 -0.63
H -0.63 -0.63  0.63

Save this file as
methane.mop,
and then run

mopac methane.mop

to perform structure optimization of methane
using the PM7 method.

Sample 3: Checking HOMO and LUMO with Formaldehyde

When learning about HOMO and LUMO,
a small organic molecule such as formaldehyde,
H₂CO,
is also an easy example to work with.

PM7 PRECISE VECTORS=(3,3)

Formaldehyde HOMO LUMO

C  0.000  0.000  0.000
O  1.210  0.000  0.000
H -0.600  0.940  0.000
H -0.600 -0.940  0.000

This calculation optimizes the structure
and also outputs molecular orbitals
around the HOMO and LUMO.

After the calculation,
check the orbital energies
and the boundary between occupied and unoccupied orbitals
in the .out file.

Specifying CHARGE

When calculating an ion,
the total charge of the molecule can be specified.

For example, for a +1 ion,

CHARGE=1

and for a -1 ion,

CHARGE=-1

are specified.

If no charge is specified,
MOPAC treats the system as neutral by default.

Calculating Solvent Effects

In MOPAC,
the COSMO model can also be used
to approximately include solvent effects.

The dielectric constant of the solvent
is specified using the EPS keyword.

For example, when approximating water
near 25°C,

PM7 EPS=78.4

can be specified.

Points to Note When Using MOPAC

MOPAC is fast and convenient calculation software,
but calculated results cannot be treated as experimental values themselves.

Semi-empirical calculation methods
have molecules for which they work well
and molecules for which they work less well,
depending on the method.

In addition, structure optimization may reach
different local minimum structures
depending on the initial structure.

What Should Students Calculate First?

When using MOPAC for the first time,
it is easier to understand the calculations
by starting with molecules containing only a small number of atoms.

For example,
calculating the HOMO and LUMO of ethylene and benzene
allows molecular orbitals derived from π-electron systems
to be compared.

Comparing HOMO and LUMO energies among different molecules
also provides practice in considering the relationship
between molecular orbitals and chemical reactivity.

Is MOPAC Suitable for Learning Molecular Orbital Calculations?

MOPAC is relatively easy to use
even for students learning molecular orbital calculations for the first time.

Because the input files are simple
and small molecules can be calculated in a short time,
it is also suitable for exercises
in which molecular structures are modified
and the results are compared.

The official MOPAC documentation also explains
that it can be used as an educational tool
for teaching physical chemistry concepts to students
and as a learning tool before using
more computationally intensive quantum chemistry software.

Summary

MOPAC is quantum chemistry calculation software
based mainly on semi-empirical molecular orbital methods.

Using calculation methods such as PM3, PM6, and PM7,
it can calculate molecular structure optimization,
heats of formation, atomic charges,
and molecular orbitals such as HOMO and LUMO.

It was previously provided as commercial software,
but it is now open source
and can be used free of charge
under the Apache License 2.0.

Installers are available for Windows, macOS, and Linux,
and MOPAC can also be installed through Conda.

The basic method of use is to create a text-based input file,
run MOPAC,
and check the results in files such as the .out file.

In particular, because HOMO, LUMO,
molecular structures, electronic states,
and other quantities can actually be calculated and examined,
MOPAC can also be used as software for learning molecular orbitals
in university-level physical chemistry and quantum chemistry courses.