For example, the first basis function in the irreducible representation is an type on the oxygen indicating the oxygen 1 orbital. Figure 3.2 . 2.0 2.0 2.0 IVO End of Input Figure 3.2 . Several techniques exist for correct allocation of electrons. As a test of the electron allocation, the energy obtained should be the same with and without symmetry.
Water is a simple case, more so when using the minimal basis set.
That is why the third number after the OCCUpied keyword is zero. Keyword AUFBau can be of great help in difficult cases but it must be used with caution because the right result is not always guaranteed. The next keyword, OCCNumbers , is not required in this case as the default occupation for each orbital is 2.0. Any number between 0.0 and 2.0 can be specified for any number of orbitals. An example input containing partially filled orbitals can be seen on section 4.1.1 .