SCF Keyword
Last Update: 06/04/2001

This keyword controls the functioning of the SCF procedure. Options are used to specify the desired behavior, alternate algorithms, and so on. See chapter 4 for more information on maximizing performance in the SCF for different problems.

Single point direct SCF calculations are run with modest convergence criteria automatically in the interest of speed. The default for this case is sufficient for 0.1 kcal mole^-1 accuracy in the SCF energy and 3 decimal places in the density matrix--sufficient for population analysis, electrostatic potential derived charges, and the like. SCF=Tight requests full convergence for this case.

SCF and DFT single point energy calculations involving basis sets which include diffuse functions should always use the SCF=Tight keyword to request tight SCF convergence criteria.

At the other extreme, sometimes it is useful to start off optimizations with less accurate integral, SCF, and CPHF cutoffs and convergence criteria and then to enable the more accurate and expensive limits only when the geometry has stabilized. The Sleazy option reduces all of these cutoff values. It also turns off archiving.

See reference [271] for a discussion of SCF convergence and stability.

Direct
Requests a direct SCF calculation, in which the two-electron integrals are recomputed as needed. This is the default SCF procedure in Gaussian 98. This is possible for all available methods, except for MCSCF second derivatives and anything using complex orbitals. Note that for single-point direct SCF calculations, a loose convergence criterion (10^-4) is used in the interest of speed.

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.

QC
Calls for the use of a quadratically convergent SCF procedure [272]. 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. It is available only for RHF closed shell and UHF open shell calculations.

DIIS
Calls for and NoDIIS prohibits use of Pulay’s Direct Inversion in the Iterative Subspace extrapolation method [273]. The default is to use DIIS for Hartree-Fock and DFT methods, but not for semi-empirical methods; it may be enabled for the MINDO3, MNDO, and AM1 methods only.

SD
Does steepest descent SCF.

SSD
Does scaled steepest descent SCF.

DM
Calls for use of the direct minimization SCF program [274]. 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.

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

PROCEDURE-RELATED OPTIONS

FON
Start the SCF calculation by doing a superposition of states (simulated annealing). This procedure partially occupies virtual orbitals in the first few iterations in order to help get to the ground state wavefunction. It is the default. The NoDIIS option disables this procedure.

VShift[=N]
Shift orbital energies by N*0.001 (i.e., N millihartrees); N defaults to 100. This option disables automatic archiving.

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
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-dependant cases. VTL is a synonym for VeryTightLinEq.

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. The default for direct SCF, 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. The default for everything except CASSCF and direct SCF single points. Synonymous with NoSinglePoint, NoSP, NoSleazy and TightIntegrals.

SinglePoint
Requests the loose SCF convergence criteria appropriate for single points. The default for single point CASSCF or direct SCF. Can be abbreviated SP. NoSinglePoint and NoSP are synonymous with Tight.

Sleazy
Reduce cutoffs for integrals, SCF, and CPHF (also disables automatic archiving). Not recommended for production-quality energies. NoSleazy is synonymous with Tight.

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.

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