Gaussian 03 Online Manual
Last update: 16 January 2007


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 must be provided by the user, is expected to:

  • Convert the text file—referred to as the "input file"—into the proper format for the external 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."


By default, the Gau_External script is passed three parameters:

$ Gau_External key InputFile OutputFile

The first argument is 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 small system of a 3-layer ONIOM (S).

The remaining two parameters are the name of the file Gaussian has prepared as input for the external program (i.e., the input file), and the name of the file which should be read in after the external program completes (i.e., the output file). Both of these files are deleted by Gaussian once the results have been recovered.

You may specify a different script by including its name as the option to the External keyword: e.g., External=MyScript. Arguments to the script may also be included: External="RunTink Amber". In either case, the specified script is also passed the parameters mentioned above as its final three arguments.


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. The remaining lines specify the atomic number, coordinates and molecular mechanics charge for each atom.


The output file is in fixed format, and has the following data:

Items Pseudo Code Line Format Units
energy dipole-moment (xyz) E, Dip(I), I=1,3 4D20.12 atomic
gradient on atom (xyz) FX(J,I), J=1,3; I=1,NAtoms 3D20.12 Hartrees/Bohr
polarizability Polar(I), I=1,6 3D20.12 atomic
dipole derivatives DDip(I), I=1,NAt9 3D20.12 atomic
force constants FFX(I), I=1,NAt3TT 3D20.12 atomic

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.


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

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


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