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Requests a calculation using an external program. This mechanism is primarily intended to facilitate the use of external programs to provide the low-level calculations in ONIOM calculations, but can also be used to conduct geometry optimizations using Gaussian's optimizer with external programs providing the function values and derivatives.

Gaussian uses a standardized interface to run an external program to produce an energy (and optionally a dipole moment or forces) at each geometry. A text file is produced with the current structure, and a script named Gau_External is run by default (see below for information on specifying an alternate script). This script, which is provided by the user, is expected to:

  • Convert the text file—referred to as the “input file”—into input for another program.
  • Run that program.
  • Convert the results into a standard text form for recovery by Gaussian. The converted file for use by Gaussian is referred to as the “output file.”

You may specify a different script by including its name as the option to the External keyword: e.g., External=MyScript.

By default, the Gau_External script is passed six parameters:

$ Gau_External layer InputFile OutputFile MsgFile FChkFile MatElFile

The parameters are defined as follows:

layer    A key letter indicating whether the computation is being performed on the real system (R), the model system of a 2-layer ONIOM or the middle layer of a 3-layer ONIOM (M), or the model system of a 3-layer ONIOM (S).
InputFile    The name of the file Gaussian has prepared as input for the external program.
OutputFile    The name of the file which should be read in after the external program completes.
MsgFile    The name of a file for messages; if the script creates this file, then its contents are copied to the Gaussian output file.
FChkFile    A formatted checkpoint file. If the appropriate options are set to link 402, then this file is created from the read-write file before starting the external script, and may be read to import results after the script finishes instead of Gaussian input being provided via OutputFile. The output formatted checkpoint file can contain an initial two blank lines plus the data to be updated in the usual format; it does not need to contain any information which is to remain unchanged.
MatElFile    Matrix element file. This is a Fortran unformatted file designed to export data such as the overlap and Core Hamiltonian matrix and two-electron integrals in an extensible format. The structured is documented in Interfacing to Gaussian 16.

All of these files are deleted by Gaussian once the results have been recovered.

Additional arguments to the script may also be included:

External="RunTink Amber"

In this example, the actual command would be:

$ RunTink Amber layer InputFile OutputFile MsgFile FChkFile MatElFile

The specified script is always passed the parameters mentioned above as its final six arguments.

Input File Format

The input file has the following format:

#atoms  derivatives-requested  charge  spin  
atomic#  x  y  z  MM-charge Repeated for each atom.

The first line specifies the number of atoms in the molecule, what derivatives are to be computed (0=energy only, 1=first derivatives, 2=second derivatives), and the molecule's charge and spin multiplicity (format 4I10). The remaining lines specify the atomic number, coordinates, and molecular mechanics charge for each atom (format I10, 4F20.12).

Output File Format

The output file is in fixed format and has the following data (all in atomic units):

Items   Pseudo Code   Line Format
energy, dipole-moment (xyz)   E, Dip(I), I=1,3   4D20.12
gradient on atom (xyz)   FX(J,I), J=1,3; I=1,NAtoms   3D20.12
polarizability   Polar(I), I=1,6   3D20.12
dipole derivatives   DDip(I), I=1,9*NAtoms   3D20.12
force constants   FFX(I), I=1,(3*NAtoms*(3*NAtoms+1))/2   3D20.12

The second section is present only if first derivatives or frequencies were requested, and the final section is present only if frequencies were requested. In the latter case, the Hessian is given in lower triangular form: αij, i=1 to N, j=1 to i. The dipole moment, polarizability, and dipole derivatives can be zero if none are available.

It is also possible to provide one-electron or one- and two-electron integrals and other matrix elements to an external program and to recover results such as MOs or densities from the other program. Full details and examples are in the g09/doc subdirectory (doc folder on Windows). Options must follow the name of the script.


A Fortran unformatted file will be provided to the external program containing coordinates and one-electron matrix elements (overlap, core Hamiltonian, etc.). Refer to g09/doc/unfdat.txt for details on the contents of the file and to g09/doc/rdmat.F for a sample program which reads the file and prints its contents. 1Elintegrals is a synonym for this option.


The Fortran unformatted file should also contain two-electron integrals. This option implies SCF=Conventional.


A formatted checkpoint file should be generated and provided to the external program.


A Fortran unformatted file will be provided to the external program and an updated or replaced file with the same structure will be read by G16 for the results, in lieu of the default text output file expected from the external program/script.


A formatted checkpoint file will be generated and provided to the external program, and a new .fchk file will be read to import results afterwards.


This option can be used to alter the content of the external text input file that Gaussian 09 automatically generates for the external script. When the data transfer between Gaussian 09 and the external script is handled using one of the options above (e.g. IOFchk), the default external text input file is not needed. With this option, a section (delimited by the usual blank lines) will be read from the Gaussian 09 input file. The text in this section will be placed in the external text input file instead of the usual content of such file. This provides additional flexibility to provide extra instructions to the external script.

Test job 769 serves as an example of these options.

External scripts may also be specified as one of the models for the ONIOM keyword (see the examples).

The Gaussian stand-alone mm utility program can be run with the -external switch, which causes it to read and write data in the formats used by the External interface.

See Interfacing to Gaussian 16 for information about exchanging data with other programs.

The following route section specifies an external script for the low layer of a 3 layer ONIOM calculation:

# ONIOM(B3LYP/6-31G(d):AM1:External="RunTink Amber") Opt

The following route section specifies an external script for the high accuracy layer of a 2 layer ONIOM job:

# ONIOM(External="RunCC SDT":B3LYP/6-31G(d)) Opt

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