17.3 Starting orbitals

The START directive can be used to specify the initial orbitals used in the SCF iteration. It is either possible to generate an initial orbital guess, or to start with previously optimized orbitals. Alternatively, one can also use a previous density matrix to construct the first fock operator.

If the START card is absent, the program tries to find suitable starting orbitals as follows:

First:

Try to read orbitals from record specified on the ORBITAL or SAVE card or the corresponding default (see ORBITAL). All files are searched.

Second:

Try to find orbitals from a previous SCF or MCSCF calculation. All files are searched.

Third:

If no orbitals are found, the starting orbitals are generated using approximate atomic densities or eigenvectors of $h$ (see below).

Since these defaults are usually appropriate, the START card is not required in most cases.