Gaussian 03 Online Manual
Last update: 9 October 2003

External

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. This script is expected to:

  • Convert the text file into input for another program.

  • Run that program.

  • Convert the results into a standard text form for recovery by Gaussian.

The script is passed two parameters: the name of the file Gaussian has prepared as input for the external program (the input file), and the name of the file which should be read in after the external program completes (the output file).

INPUT FILE FORMAT

The input file has the following format:

#atoms   derivatives-requested    charge & spinlow    charge & spinmedium    charge & spinhigh 
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.

OUTPUT FILE FORMAT

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

energy   dipole-moment(xyz)                                 Format: 4D20.12 
force on atom (xyz)                                              Format: 3D20.12  Repeated for each atom. 
Hessian (xyz)                                                       Format: 3D20.12  Repeated as needed.

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.