Quantum Chemistry Laboratory | |
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Prof. N. Rösch |

- Atom symbols are designated by the usual element symbols
`H`,`He`,`Li`,`Be`,`B`, etc. by and the following special symbols*Dummy atoms*(symbol`X`) facilitate the definition of a complicated molecular structure. They neither carry a positive charged nucleus nor a basis set (AO). In the electronic structure calculation proper, they are totally ignored. Examples for the advantageous use of dummy atoms will be provided in Exercise 5.*Ghost atoms*(symbol`Bq`) have no positively charged nucleus, but the do carry atomic orbitals. For example, they are used to define*bond orbitals*, i.e. orbitals centered at the middle of a bond. Another important usage is when carrying out a counterpoise correction to estimate the__B__asis__S__et__S__uperposition__E__rror (BSSE).

- The bond distance N1-N2 is that between atoms number N1 and atom number N2. Any previously defined atom N2 can be specified.
- The angle N1-N2-N3 defines the bond angle a located at atom number N2. The atoms N1 and N3 lie at the two sides of the angle. The allowed range for bond angles is 0° < a < 180°.
- The dihedral angle N1-N2-N3-N4 specifies the angle between the two planes [N1,N2,N3] and [N2,N3,N4]. The value of a dihedral angle has to fall in the range from -180° to +180°.

Have a look at the example hydrogen peroxide H_{2}O_{2}.

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