Gaussian 90 File Reader
John R. Murray
April 22, 1993

Summary: File reader for Gaussian 90 density files

Note: from briefly perusing the documentation for G92, I believe this file
reader will also read electrostatic potential files, which are the same
format as density files.

The "Gaussian 90" file reader is somewhat misnamed, as it was only written
to read density grids (as produced by the DENSITY keyword, I believe) and
not density+gradient or density+gradient+divergence grids, or other g90
files. (note: it should be able to read any g90 files which have the same
format as density files, i.e. begin with the same format of grid definition
and atom data, followed by exactly N1*N2*N3 data points (1 scalar value per
grid point)) Also, I don't think I ever got handling of arbitrarily skewed
grids working properly - if things seem to be off position, try a grid with
orthogonal axes aligned with the x, y and z grids - for example for a
20x20x20 grid with origin at 0,0,0 the first four lines could look like:

NAtoms 0 0 0
20 .02 0 0
20 0 .03 0
20 0 0 .04


where '.02', '.03' and '.04' are some increments I pulled out of a hat.
(the important thing is that 2 of the 3 step sizes be zero)

If you name G90 files 'foo.g90' the format will be pre-set to G90 for you.
timebased density grids currently must be put in different files, named
something like 'foo@23.g90'

Once a G90 file is read in, several data objects are created - one 3d scalar
dataset corresponding to the density data, and a geometry dataset containing
spheres representing the atoms. For a file called foo.g90, if another
file called foo.g90.bond exists, SciAn will attempt to read it for a list
of bonds between atoms, which will appear as another geometry dataset
containing cylinders representing the bonds. The format of this file is just
pairs of atom numbers between which you wish a bond cylinder to appear.
Atoms are implicitly numbered by the order in which they appear in the grid
file, so if, for example, the first six atoms listed in the grid file are
6 carbons in a ring, the first six lines of the .bond file might be:

1 2
2 3
3 4
4 5
5 6
6 1

Yes, the G90 file reader is a very limited and messy beast. It was written
on the spot at the request of a single local scientist, who has since left
SCRI. Other local people have begun to use it (and therefore to ask me for
fixes/improvements) and a more robust version of a G90/G92 reader is in
the works. If you have suggestions for improvement or would like to see
particular Gaussian file formats included in the reader, please send mail.

John R. Murray