16 THE SCF PROGRAM

`HF`or`RHF`- calls the spin-restricted Hartree-Fock program
`UHF`or`UHF-SCF`,*options*- calls the spin-unrestricted Hartree-Fock program

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.**

- 16.1 Options
- 16.2 Defining the wavefunction
- 16.3 Saving the final orbitals
- 16.4 Starting orbitals
- 16.5 Rotating pairs of orbitals
- 16.6 Using additional point-group symmetry
- 16.7 Expectation values
- 16.8 Polarizabilities
- 16.9 Miscellaneous directives
- 16.10 Handling difficult cases: When SCF does not converge
- 16.11 Advanced use: Core-excited states