Gaussian 03W Bonding: How Does It Work?

[dreamzndigita]

I'm a new Gaussian 03W user and I have what I assume is a pretty basic question.

How does Gaussian "know" how atoms are bonded to each other when a molecule is specified in an input file. An example molecule specification in Cartesian coordinates follows:

0 1
C 0.00 0.00 0.00
C 0.00 0.00 1.52
H 1.02 0.00 -0.39
H -0.51 -0.88 -0.39
H -0.51 0.88 -0.39
H -1.02 0.00 1.92
H 0.51 -0.88 1.92
H 0.51 0.88 1.92

I know that the geom=connect option can be used to explicitly set up bonds but it doesn't seem to be necessary. Thanks!

 

[chemisttree]

It probably calculates distances between each atom and all of the other atoms. Only one will be within bonding distance and it chooses that one. A set of definitions may prevent H-H bonds in favor of C-H or O-H, etc...

In the case of the two carbons,

C 0.00 0.00 0.00
C 0.00 0.00 1.52

you will notice that one carbon lies at the center of the coordinate system (0.00, 0.00, 0.00) and one lies 1.52 (angstroms?) along the Z-axis. These two carbons are 1.52 angstroms(?) apart. A lookup table for C-C bonds is likely used to determine that these two are within bonding distance from each other.