SCF Section

The SCF section controls the Hartree–Fock self-consistent field procedure.

Keywords

Keyword	Type	Default	Description
`Algorithm`	`rhf` \| `uhf` \| `rohf`	from `task:`	SCF method
`Guess`	`core` \| `huckel` \| `sad` \| `read`	`sad`	Initial MO guess
`MaxCycle`	integer	100	Maximum SCF iterations
`Conver`	integer	8	Convergence threshold = 10⁻ᴺ
`Diis`	integer	6	DIIS subspace size
`NoDiis`	boolean	false	Disable DIIS
`Shift`	float	0.0	Static level shift (Hartree)
`VShift`	boolean	false	Dynamic virtual orbital level shift
`Damp`	float	0.0	Density damping factor
`NoDamp`	boolean	false	Turn damping off
`Fermi`	float	0.0	Fermi broadening temperature (Hartree)
`Direct`	boolean	false	Direct SCF (recompute ERIs each iteration)
`Save`	boolean	false	Write checkpoint after SCF
`Restart`	boolean	false	Read initial guess from checkpoint
`Tight`	boolean	false	Tighter convergence preset
`SD`	boolean	false	Steepest Descent SCF
`SSD`	boolean	false	Scaled Steepest Descent SCF
`MaxRot`	integer	512	Max QC-SCF macroiterations
`MaxNr`	float	0.01	NR gradient threshold
`FullLinear`	boolean	false	Full 1D line search in QC-SCF
`OldQC`	boolean	false	Old polynomial-only QC line search

Standard RHF:

SCF
  Algorithm  rhf
  MaxCycle   100
  Conver     8
END

Tight convergence with custom DIIS:

SCF
  MaxCycle 200
  Conver   10
  Diis     10
END

Level shift for difficult convergence:

SCF
  Shift    0.5
  MaxCycle 150
END

Direct SCF for large molecules:

SCF
  Direct   true
  MaxCycle 100
END

Checkpointing:

SCF
  Save     true
  MaxCycle 100
END

To restart: set Restart true in a follow-up job.

The Algorithm keyword in SCF is typically redundant with the task: line (e.g., task: RHF STO-3G), but can be used to override it.
Guess sad (Superposition of Atomic Densities) is the default and recommended initial guess for most systems.
Direct SCF recomputes ERIs each iteration, trading memory for CPU time.
QC-SCF is a second-order optimization method that directly minimizes the energy with respect to orbital rotations. Use XQC for DIIS-first with QC fallback.