Up: Miscellany
The GAUSSIAN command is used to invoke the Gaussian program1 Currently, this interface may only be used to calculate partial charges for fragments of the system. It uses the Gaussian 94 program to calculate the wavefunction and electrostatic field for the fragment. Four different methods are provided for calculating partial charges from the wavefunction. All of these methods determine partial charges by performing a least squares fit of the potential generated by the partial charges to the potential calculated using the wavefunction. The fundamental difference between the methods is the layout of points where the electrostatic potential is determined by the wavefunction, and subsequently used for the least squares fit of atomic charges.
The first method, PDM, uses two programs written by Don Williams, PDM88 and PDGRID.23 The PDGRID program lays out a grid of points around the fragment where the potential will be calculated, and the PDM88 program does the least squares fit to determine the best values for the partial charges.4 The other methods have been incorporated directly into Gaussian 94, and use different grid layouts. There is the scheme due to Merz, Singh, and Kollman,56 identified by the keyword, MK; the scheme due to Chirlian and Francl,7 identified by the keyword, CHELP; and the scheme to Breneman and Wiberg,8 identified by the keyword, CHELPG.
All of these schemes have their own values for van der Waals radii encoded within them. However, the default in this interface is to use the radii from the parameters in CONGEN. If you want to use the radii in the external programs, use the EXTRADII keyword.
This command simplifies the use of these three programs. You specify the atoms you want charges for, and the programs are invoked in turn to calculate the charges. Remember that the time for the calculation increases approximately with the fourth power of the number of electrons. A number of files are generated when this command is executed. Normally, these files are deleted after the command is complete, but you can request that they be saved.
Note that the collection of atoms that you specify should be a complete molecule including hydrogens. It does not make physical sense to do anything different, although the program does not check for completeness.
If you use this command to calculate results that are eventually published, please ensure that both Gaussian 94 and the fitting scheme you use is properly referenced.
GAUSsian CHARges {SELECT atom-selection END}
[BASIs word] [SCF word] [TOTAl real] [MEMOry int]
[UNIT unit] [PREFix word] [EXTRadii] [DIPOle]
[PDM [UNDEr real] [SHELl real] [SPACing real] ]
[MK ]
[CHELP ]
[CHELPG ]
[STEPs repeat(step-options) END]
[SAVE] [NORUn]
[MERGe atom-selection END]
repeat( AVERage atom-selection END )
[ ALL ]
[ NONE ]
[ [NO]CREAte ]
[ [NO]HF ]
step-option ::= [ [NO]GRID ]
[ [NO]POTEntial ]
[ [NO]FIT ]
[ [NO]SCAN ]
[ [NO]DELEte ]
See Atom Selection, for the syntax of an atom-selection.
The GAUSSIAN command functions by writing a set of input files for Gaussian, PDGRID, and PDM88; preparing a Bourne shell script to execute each program in turn; executing the script; and reading the results. The options are interpreted as follows:
The following table gives the file types for all the intermediate files used:
[1] Gaussian 94, Revision E.2, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995.
[2] D.E. Williams, Quantum Chemistry Program Exchange, Program 568; PDM88 (which includes PDGRID)
[3] D. E. Williams, “Representation of the Molecular Electrostatic Potential by Atomic Multipole and Bond Dipole Models”, J. Comput. Chem. 9, 745-763 (1988).
[4] The program, PDM88, is obsolete. Although it serves the need for a charge calculation in CONGEN, the newer version has more features for those users interested in exploring charge calculations. For further information please contact Dr. Donald E. Williams, Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA, Tel: (502)588-5975, Fax: (502)588-8149, E-mail: dewill01@ulkyvx.bitnet.
[5] U. C. Singh and P. A. Kollman, J. Comput. Chem., 5, 129 (1984).
[6] B. H. Besler, K. M. Merz, and P. A Kollman, J. Comput. Chem., 11, 431 (1990).
[7] L. E. Chirlian and M. M. Francl, J. Comput. Chem., 8, 894 (1987).
[8] C. M. Breneman and K. B. Wiberg, J. Comput. Chem., 11, 361 (1990).