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Homework Help: Gaussian 03W Bonding

  1. Feb 6, 2009 #1
    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!
  2. jcsd
  3. Feb 9, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    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.
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