1.11.2 Route section
The route section of a Gaussian03 input file specifies the type of calculation to be performed. According to the Gaussian03 User’s Reference [Frisch and Frisch 1999] there are three key components to this specification: - a) the job type, b) the method and c) the basis set. Optimization (opt keyword) and frequency calculation (freq keyword) are examples of job types. The combination of method and basis set specifies a model chemistry to Gaussian03, specifying a level of theory. HF (Hartree Fock), MPn (Moller Plesset Perturbation Theory), DFT(Density Functional Theory) are examples of methods. If no method keyword is specified, HF is assumed as a default. Most methods require a basis set. If no basis set keyword is specified then STO-3G basis will be used. STO-3G, 3-21G, 6-21G, and 6-31G are examples of basis sets. Single first polarization functions can also be requested using the usual * or ** notation. 6-31G* (or 6-31G(d)) is 6-31G with added d polarization functions on non-hydrogen atoms; 6-31G** (or 6-31G(d,p)) is 6-31G* plus p polarization functions for hydrogen. The + and ++ diffuse functions are available with some basis sets. 6-31+G is 6-31G plus diffuse s and p functions for non-hydrogen atoms; 6-31++G has diffuse functions for hydrogen also. For example, 6-31G(3df, 2p) designates the 6-31G basis set supplemented by diffuse functions, 3 sets of d functions and one set of f functions on heavy atoms, and supplemented by 2 sets of p functions on hydrogens. Which basis set to use is related to the objective of the calculation and the molecules to be studied. Even a large basis set is not always a guarantee for agreement with experimental data.
1.11.3 Z-matrix
The molecular structure given by Z-matrix definition contains information on the type of atoms used and their geometrical positions. Z-matrix, defines a molecule atom by atom in terms of bond lengths, bond angles, and dihedral angles. Z-matrix neither defines the bonds to be formed in the calculation nor represents a given electronic state. [Clark 1985] discusses ways of efficiently writing the Z-matrix, the use of symmetries in the molecule and the use of dummy atoms. The text also contains an excellent explanation of how quantum chemical computational programs work.
For visualizing as well as specifying the Z-matrix MOLDEN [Schaftenaar 2000] visualization package can be used. It automatically assigns initial values to the bond lengths, bond angles, and dihedral angles.
The charge is the overall charge (in atomic units) on the system specified in the Z-matrix. The individual electronic spins add vectorially to give a total electronic spin S whose magnitude has the possible values [S(S+1)]½ħ with S = 0, ½, 1, ... The quantity 2S+1 is called the multiplicity. The multiplicity is a measure of the number of unpaired electrons in the system. It is generally computed using the following thumb rule:
|
(1.40) |
where, nu is the number of unpaired electrons.