Often, no further input is necessary. By default, the number of electrons is equal to the nuclear charge, the spin multiplicity is 1 (singlet) for an even number of electrons and 2 (doublet) otherwise, and the wavefunction is assumed to be totally symmetric (symmetry 1) for singlet calculations. The Aufbau principle is used to determine the occupation numbers in each symmetry. Normally, this works well in closed-shell and many open-shell cases, but sometimes wrong occupations are obtained. In such cases, the OCC and/or CLOSED directives can be used to force convergence to the desired state. The default behaviour can be modified either by options on the command line, or by directives.
In open-shell cases, we recommend to use the WF, OCC, CLOSED, or OPEN cards to define the wavefunction uniquely. Other commands frequently used are START and ORBITAL (or SAVE) to modify the default records for starting and optimized orbitals, respectively. The SHIFT option or directive allows to modify the level shift in the RHF program, and EXPEC to calculate expectation values of one-electron operators (see section 6.13). Section 16.10 discusses strategies for dealing difficult molecules and convergence problems.
Density fitting can be used for closed and open-shell spin-restricted HF and is invoked by a prefix DF- (DF-HF or DF-RHF, see section 15). For UHF, only Coulomb fitting is possible (CF-UHF). Density fitting very much speeds up calculations for large molecules. The greatest savings are seen for large basis sets with high angular momentum functions. For details see R. Polly, H.-J. Werner, F. R. Manby, and Peter J. Knowles, Fast Hartree-Fock theory using local density fitting approximations, Mol. Phys. 102, 2311 (2004). All publications resulting from DF-HF or DF-KS calculations should cite this work.