4.3.1 Introduction

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:

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  Addition of several useful SCF convergence algorithms and support for user-specified hybrid algorithm (Section 4.5.12).

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  More general and user-friendly internal stability analysis and automatic correction for the energy minimum (Section 4.5.17).

GEN_SCFMAN also supports a wider range of orbital types, including complex orbitals. A full list of supported orbitals is:

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  Restricted (R): typically appropriate for closed shell molecules at their equilibrium geometry, where electrons occupy orbitals in pairs.

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  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.

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  Restricted open-shell (RO): for open-shell molecules, where the $\alpha$ and $\beta$ orbitals are constrained to be identical.

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  Canonical restricted open-shell (can. RO or “ROSCF-DODS”): canonical form of ROSCF, see Section 4.5.18 for details.

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  Open-shell singlet ROSCF (OS_RO): see the “ROKS" method documented in Section 7.7.4.

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  Generalized (G): i.e., each MO is associated with both $\alpha$ and $\beta$ spin components.

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  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.

Starting Q-Chem v7.0, MPI+OMP-parallel non-RI SCF calculations done with DIIS (Section 4.5.3), GDM (Section 4.5.7), and the robust SCF pipelines (Section 4.5.13) are supported for RHF, UHF, and ROHF ansätze. Q-Chem v7.0 uses the replicated-Fock approach in which each MPI rank allocates memory for the entire Fock matrix. Since the full Fock matrix is built at the end of the Fock build during the SCF iteration, the per-rank memory overhead must be considered while spawning multiple MPI ranks on the same physical node.