As of version 5.1, Q-Chem uses a new SCF package, GEN_SCFMAN, developed by E. J. Sundstrom, P. R. Horn and many other coworkers. In addition to supporting the basic features of the previous SCF package (e.g. restricted, unrestricted and restricted open-shell HF/KS-DFT calculations), many new features are now available in Q-Chem, including:
GEN_SCFMAN also supports a wider range of orbital types, including complex orbitals. A full list of supported orbitals is:
Restricted (R): typically appropriate for closed shell molecules at their equilibrium geometry, where electrons occupy orbitals in pairs.
Unrestricted (U): - appropriate for radicals with an odd number of electrons, and also for molecules with even numbers of electrons where not all electrons are paired, e.g., stretched bonds and diradicals.
Restricted open-shell (RO): for open-shell molecules, where the and orbitals are constrained to be identical.
Open-shell singlet ROSCF (OS_RO): see the “ROKS" method documented in Section 7.8.2.
Generalized (G): i.e., each MO is associated with both and spin components.
The use of complex orbitals (with Hartree-Fock only): restricted (CR), unrestricted (CU), and generalized (CG).
Aspects of an SCF calculation such as the SCF guess, the use of efficient algorithms to construct the Fock matrix like occ-RI-K (see Section 4.6.8), are unaffected by the use of GEN_SCFMAN. Likewise, using GEN_SCFMAN does not make any difference to the post-SCF procedures such as correlated methods, excited state calculations and evaluation of molecular properties.
It should be noted that many special features (e.g. dual-basis SCF, CDFT, etc.) based on Q-Chem’s old SCF code are not yet supported in GEN_SCFMAN. They will become available in the future.