The default SCF procedure uses a combination of EDIIS [Kudin02] and CDIIS, with no damping or Fermi broadening. In Gaussian 16, 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 calls for and NoDIIS prohibits use of Pulay’s Direct Inversion in the Iterative Subspace (DIIS) extrapolation method [Pulay82].
Use only CDIIS. CDIIS implies Damp as well.
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.
Allow dynamic damping for up to N SCF iterations (the default is 10).
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.
Add an extra SCF=QC step in case the first-order SCF has not converged. XQC defaults to MaxConventional=32
Provides a new algorithm that is useful for difficult SCF convergence cases involving very large molecules. It does steepest descent and then scaled steepest descent as in QC, but then switches to regular SCF instead of quadratic convergence, using the quadratic algorithm only if the regular SCF fails to converge. YQC defaults to MaxConventional=32
Sets the limit on conventional SCF cycles during SCF=XQC and SCF=YQC to N.
Use pseudo-diagonalization in Link 502 whenever possible, with full diagonalization only at the early cycles, at the end, and every Nth cycle in between. PDiag is a synonym for this option. This is the default for semi-empirical methods (the default is N=30).
Does steepest descent SCF
Does scaled steepest descent SCF.
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.
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.
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.
Use tight convergence in linear equation solution throughout SCF=QC. By default, the convergence criterion is tightened up as the rotation gradient is reduced.
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
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.
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.
Options Related to Convergence and Cutoffs
Specifies N points per Angstrom^2 (N>0) or -N tesserae (N<0). The default is 5.
Sets the SCF convergence criterion to 10-N. SCF convergence requires both <10-N RMS change in the density matrix and <10-(N-2) maximum change in the density matrix. Note that the energy change is not used to test convergence; however, an SCF 10-N RMS density matrix change typically corresponds to a 10-2N change in energy in atomic units. For GVB and CASSCF calculations, SCF convergence is determined not by change in the density matrix, but rather in terms of the orbital change and energy change, respectively.
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.
Use normal, tight convergence in the SCF. This is the default. Synonymous with TightIntegrals.
Turns off optional O(N3) steps to speed up very large calculations (>5000 basis functions).
Sets the maximum rotation gradient for a Newton-Raphson step in SCF=QC and SCF=YQC to 10-N. Below this threshold the program switches to the QC SCF procedure (if using SCF=QC), or to the regular SCF procedure (if using SCF=YQC). Above this, scaled steepest descent is used; above 100 times this, steepest descent is used. The default value for N is 2.
Symmetrize the density matrix at every SCF iteration to match the symmetry of the molecule (“density symmetrize”). NoDSymm is the default. DSymm implies IDSymm.
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.
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).
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 the SCF from the checkpoint file. SCF=DM cannot be restarted. SCF=Restart skips steps which are not necessary when restarting an SCF calculation, but which are necessary when reading in a guess from a calculation with a different basis set or at a different geometry. In contrast, if you want to start a new SCF using the restart information from a calculation with a different geometry and/or a different basis set, use Guess=Restart.
Last updated on: 05 January 2017. [G16 Rev. C.01]