20.5.1 Defining the starting guess

START,record,[options];

record: dump record containing starting orbitals. As usual, record has the form irec.ifil, where irec is the record number (e.g., 2140), and ifil the file number (usually 2). The options can be used to select orbitals of a specific type; for details, see section 2.16.

If this card is missing, the program tries to find suitable starting orbitals as follows:

First:

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

Second:

Try to find orbitals from the most recent MCSCF calculation. All files are searched.

Third:

Try to find orbitals from the most recent SCF calculation. All files are searched.

If no orbitals are found, a starting orbital guess is generated.

It is often useful to employ MCSCF orbitals from a neighbouring geometry as starting guess (this will happen automatically if orbitals are found, see the above defaults). Note, however, that frozen-core orbitals should always be taken from an SCF or MCSCF calculation at the present geometry and must be specified separately on the CORE card. Otherwise the program is likely to stop with error ``non-orthogonal core orbitals''. The program remembers where to take the core orbitals from if these have been specified on a CORE card in a previous MCSCF calculation.