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:
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 four parameters:
$ Gau_External layer InputFile OutputFile MsgFile
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. |
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
The specified script is always passed the parameters mentioned above as its final four 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 (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.
External scripts may also be specified as one of the models for the ONIOM keyword (see examples 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
Last updated on: 10 May 2009