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 (Sect. 4.5.12).

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  More general and user-friendly internal stability analysis and automatic correction for the energy minimum (Sect. 4.5.16).

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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  Open-shell singlet ROSCF (OS_RO): see the “ROKS" method documented in Section 7.9.3.

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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.