DESCRIPTION

This keyword controls the functioning of the SCF procedure. Options are used to specify the desired behavior, alternate algorithms, and so on. See Efficiency Considerations for more information on maximizing performance in the SCF for difficult problems.

The default SCF procedure uses a combination of EDIIS [Kudin02] and CDIIS, with no damping or Fermi broadening. In Gaussian 09, SCF=Tight is the default.

The SCF=QC option is often helpful with difficult conversion cases. For difficult-to-converge ROHF wavefunctions, where QC cannot be used, add Use=L506 to the route section.

See reference [Schlegel91a] for a discussion of SCF convergence and stability.

ALGORITHM SELECTION OPTIONS

DIIS
DIIS calls for and NoDIIS prohibits use of Pulay’s Direct Inversion in the Iterative Subspace (DIIS) extrapolation method [Pulay82].

CDIIS
Use only CDIIS. CDIIS implies Damp as well.

Fermi
Requests temperature broadening during early iterations [Rabuck99], combined with CDIIS and damping. NoFermi suppresses Fermi broadening and is the default. By default, Fermi also implies Damp and also includes level shifting.

Damp
Turn on dynamic damping of early SCF iterations. NoDamp is the default. However, damping is enabled if SCF=Fermi or SCF=CDIIS is requested. Note that damping and EDIIS do not work well together.

NDamp=N
Allow dynamic damping for up to N SCF iterations (the default is 10).

QC
Calls for the use of a quadratically convergent SCF procedure [Bacskay81]. By default this involves linear searches when far from convergence and Newton-Raphson steps when close (unless the energy goes up). This method is slower than regular SCF with DIIS extrapolation but is more reliable. SCF=QC is not available for restricted open shell (RO) calculations.

XQC
Add an extra SCF=QC step in case the first-order SCF has not converged.

MaxConventionalCycles=N
Sets the limit on conventional SCF cycles during SCF=XQC to N.

PseudoDiagonalization=N
Use pseudo-diagonalization in Link 502 whenever possible, with full diagonalization only at the early cycles, at the end, and every N^th cycle in between. PDiag is a synonym for this option. This is the default for semi-empirical methods (the default is N=30).

FullDiagonalization
Forces full diagonalization in Link 502. This is the default for HF and DFT. FDiag is a synonym for this option.

SD
Does steepest descent SCF

SSD
Does scaled steepest descent SCF.

SaveKPoint
Save k-point information at the conclusion of the SCF. NoSaveKPoint says not to save this data, and it is the default except for numerical frequency calculations for which SaveKPoint is the default.

DM
Calls for use of the direct minimization SCF program [Seeger76]. It is usually inferior to SCF=QC and retained for backwards compatibility and as a last resort. Available only for RHF closed shell and UHF open shell calculations.

VShift[=N]
Shift orbital energies by N*0.001 (i.e., N milliHartrees); N defaults to 100. This option disables automatic archiving. N=-1 disables level shifting; NoVShift is equivalent to this setting.

MaxCycle=N
Changes the maximum number of SCF cycles permitted to N; the default is 64 (or 512 for SCF=DM and SCF=QC). Note that with DIIS turned on, memory requirements increase with increasing maximum number of cycles.

FullLinear
Specifies that L508 (SCF=QC, SD, or SSD) should do full linear searches at each iteration. By default, a full minimization is done only if the initial microiteration caused the energy to go up.

MaxRot=N
Set the maximum rotation gradient for a Newton-Raphson step in SCF=QC to 10^-N. Above this, scaled steepest descent is used; above 100 times this, steepest descent is used. The default value for N is 2.

FinalIteration
FinalIteration performs and NoFinalIteration prevents a final non-extrapolated, non-incremental iteration after an SCF using DIIS or a direct SCF has converged. The default is NoFinalIteration.

IncFock
Forces use of incremental Fock matrix formation. This is the default for direct SCF. NoIncFock prevents the use of incremental Fock matrix formation, and it is the default for conventional SCF.

Pass
For in-core calculations, saves the integrals on disk as well, to avoid recomputing them in Link 1002. Only useful for frequency jobs in conjunction with SCF=InCore. NoPass forces integrals to be recomputed during each in-core phase.

TightLinEq
Use tight convergence in linear equation solution throughout SCF=QC. By default, the convergence criterion is tightened up as the rotation gradient is reduced.

VeryTightLinEq
Use even tighter convergence in the linear equation solutions (microiterations) throughout the QCSCF. This option is sometimes needed for nearly linearly-dependent cases. VTL is a synonym for VeryTightLinEq.

INTEGRAL STORAGE OPTIONS

Direct
Requests a direct SCF calculation, in which the two-electron integrals are recomputed as needed. This is the default SCF procedure in Gaussian. This is possible for all available methods, except for MCSCF second derivatives and anything using complex orbitals.

InCore
Insists that the SCF be performed storing the full integral list in memory. This is done automatically in a direct SCF calculation if sufficient memory is available. SCF=InCore is available to force in-core storage or abort the job if not enough is available. NoInCore prohibits the use of the in-core procedure, for both the SCF and CPHF.

Conventional
The two-electron integrals are stored on disk and read-in each SCF iteration. NoDirect is a synonym for Conventional.

OPTIONS RELATED TO CONVERGENCE AND CUTOFFS

Conver=N
Sets the SCF convergence criterion to 10^-N. This is a density-based convergence criterion except for GVB and CASSCF, for which it is in terms of the orbital change and energy change, respectively.

VarAcc
Use modest integral accuracy early in direct SCF, switching to full accuracy later on. This is the default for direct SCF, and it can be turned off via NoVarAcc. VarInt is a synonym for VarAcc, and NoVarInt is a synonym for NoVarAcc.

Tight
Use normal, tight convergence in the SCF. This is the default. Synonymous with NoSinglePoint, NoSP, NoSleazy and TightIntegrals.

Sleazy
Requests the loose SCF convergence criteria appropriate for single points; equivalent to SCF=(Conv=4,VarInt,NoFinal,Direct). The default for single point CASSCF or direct SCF. Can be abbreviated SP. SinglePoint is a synonym for Sleazy.

VerySleazy
Reduce cutoffs even further; uses Int=CoarseGrid and single-point integral accuracy during iterations, followed by a single iteration with the usual single point grid (MediumGrid). Not recommended for production quality calculations.

SYMMETRY-RELATED OPTIONS

IDSymm
Symmetrize the density matrix at the first iteration to match the symmetry of the molecule (“initial density symmetrize”). NoIDSymm is the default.

DSymm
Symmetrize the density matrix at every SCF iteration to match the symmetry of the molecule (“density symmetrize”). NoDSymm is the default. DSymm implies IDSymm.

NoSymm
Requests that all orbital symmetry constraints be lifted. It is synonymous with Guess=NoSymm and Symm=NoSCF.

Symm
Retain all symmetry constraints: make the number of occupied orbitals of each symmetry type (abelian irreducible representation) match that of the initial guess. Use this option to retain a specific state of the wavefunction throughout the calculation. It is the default only for GVB calculations.

IntRep
Calls for the SCF procedure to account for integral symmetry by replicating the integrals using the symmetry operations. Allows use of a short integral list even if the wavefunction does not have the full molecular symmetry. Available for L502 (the default for RHF, ROHF and UHF) and L508 (SCF=QC).

FockSymm
Calls for the SCF procedure to account for integral symmetry (use of the petite integral list) by symmetrizing the Fock matrices. This is the default. FSymm is a synonym for FockSymm.

RESTART-RELATED OPTIONS

Save
Save the wavefunction on the checkpoint file every iteration, so the SCF can be restarted. This is the default for direct SCF. NoSave suppresses saving the wavefunction.

Restart
Restart the SCF from the checkpoint file. SCF=DM cannot be restarted.