17 THE SCF PROGRAM

The Hartree-Fock self-consistent field program is invoked by one of the following commands:

HF or RHF

calls the spin-restricted Hartree-Fock program

UHF or UHF-SCF

calls the spin-unrestricted Hartree-Fock program

In contrast to older versions of molpro, the HF and RHF directives have identical functionality and can both be used for closed-shell or open-shell calculations. Other aliases are HF-SCF or RHF-SCF.

Often, no further input is necessary. By default, the number of electrons is equal to the nuclear charge, the wavefunction is assumed to be totally symmetric (symmetry 1), and the spin multiplicity is 1 (singlet) for an even number of electrons and 2 (doublet) otherwise. The Aufbau principle is used to determine the occupation numbers in each symmetry. Normally, this works well in closed-shell cases, but sometimes wrong occupations are obtained or the wavefunction alternates between different orbital spaces. In such cases, the OCC directive must be used to force convergence to the desired state.

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 directive allows to modify the level shift in the RHF program, and EXPEC to calculate expectation values of one-electron operators (see section 4.13).