How to Solve SCF Convergence Problems in Gaussian Computational Chemistry Program

SCF

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 Nth 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=QCSD, 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=InCoreNoPass 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.

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 NoVarAccVarInt 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 NoSinglePointNoSPNoSleazy 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 SPSinglePoint 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.

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.

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.


Sample Gaussian Input Files

Here are a number of sample Gaussian input files. All geometries have been optimized at the RHF/3-21G level of theory.
-----Cut Here-----
# RHF/3-21G

Dooh H2

0 1
H1
H2 H1 r1

r1 0.7348


-----Cut Here-----
# RHF/3-21G

Coov LiH

0 1
Li1
H1  Li1 r1

r1 1.6396


-----Cut Here-----
# RHF/3-21G

Dooh BeH2

0 1
X1
Be1 X1  1.
H1  Be1 r1 X1 90.
H2  Be1 r1 X1 90. H1 180.

r1 1.3393


-----Cut Here-----
# RHF/3-21G

D3h BH3

0 1
B1
H1 B1 r1
H2 B1 r1 H1 120.
H3 B1 r1 H2 120. H1 180.

r1 1.1877


-----Cut Here-----
# RHF/3-21G

Td CH4

0 1
C1
H1 C1 r1
H2 C1 r1 H1 109.4712
H3 C1 r1 H1 109.4712 H2  120.
H4 C1 r1 H1 109.4712 H2 -120.

r1 1.0829


-----Cut Here-----
# RHF/3-21G

D*h CHCH

0 1
X1
C1 X1 r1
C2 X1 r1 C1  60.
H1 C1 r2 X1 120. C2 180.
H2 C2 r2 X1 120. C1 180.

r1 1.1876
r2 1.0509


-----Cut Here-----
# RHF/3-21G

D2h CH2CH2

0 1
C1
C2 C1 r1
H1 C1 r2 C2 a1
H2 C1 r2 C2 a1 H1 180.
H3 C2 r2 C1 a1 H1 180.
H4 C2 r2 C1 a1 H1   0.

r1   1.3149
r2   1.0736
a1 121.9078


-----Cut Here-----
# RHF/3-21G

D3d CH3CH3

0 1
C1
C2 C1 r1
H1 C1 r2 C2 a1
H2 C1 r2 C2 a1 H1  120.
H3 C1 r2 C2 a1 H1 -120.
H4 C2 r2 C1 a1 H1   60.
H5 C2 r2 C1 a1 H4  120.
H6 C2 r2 C1 a1 H4 -120.

r1   1.5426
r2   1.0841
a1 110.7913


-----Cut Here-----
# RHF/3-21G

C3v NH3

0 1
X1
N1 X1 1.
H1 N1 r1 X1 a1
H2 N1 r1 X1 a1 H1  120.
H3 N1 r1 X1 a1 H1 -120.

r1   1.0026
a1 106.3601


-----Cut Here-----
# RHF/3-21G

C*v CHN

0 1
X1
C1 X1 1.
N2 C1 r1 X1 90.0
H1 C1 r2 X1 90.0 N2 180.

r1 1.0502
r2 1.1372


-----Cut Here-----
# RHF/3-21G

Cs CH2NH

0 1
N1
C2 N1 r1
H1 N1 r2 C2 hnc1
H2 C2 r3 N1 hcn1 H1 180.
H3 C2 r4 N1 hcn2 H1   0.

r1   1.2565
r2   1.0147
r3   1.0747
r4   1.0802
a1 114.8980
a2 119.2306
a3 125.2900


-----Cut Here-----
# RHF/3-21G

Cs CH3NH2

0 1
N1
C2 N1 r1
H1 C2 r2 N1 a1
H2 C2 r3 N1 a2 H1  d1
H3 C2 r3 N1 a2 H1 -d1
H4 N1 r4 C2 a3 H1  d2
H5 N1 r4 C2 a3 H1 -d2

r1   1.4716
r2   1.0901
r3   1.0824
r4   1.0034
a1 114.7876
a2 108.9413
a3 113.6477
d1 121.4232
d2  64.2647


-----Cut Here-----
# RHF/3-21G

C2v OH2

0 1
O1
H1 O1 r1
H2 O1 r1 H1 a1

r1   0.9666
a1 107.6712


-----Cut Here-----
# RHF/3-21G

Cs CH3OH

0 1
O1
C2 O1 r1
H1 O1 r2 C2 a1
H2 C2 r3 O1 a2 H1 180.
H3 C2 r4 O1 a3 H1   d1
H4 C2 r4 O1 a3 H1  -d1

r1   1.4411
r2   0.9658
r3   1.0787
r4   1.0850
a1 110.3279
a2 106.2600
a3 112.2625
d1  61.4330


-----Cut Here-----
# RHF/3-21G

C2v CH2O

0 1
O1
C2 O1 r1
H1 C2 r2 O1 a1
H2 C2 r2 O1 a1 H1 180.

r1   1.0832
r2   1.2069
a1 122.5305


-----Cut Here-----
# RHF/3-21G

Cs CHOOH

0 1
C1
O1 C1 r1
O2 C1 r2 O1 a1
H1 C1 r3 O1 a2 O2 180.
H2 O2 r4 C1 a3 O1 180.

r1   1.1918
r2   1.3543
r3   1.0820
r4   0.9642
a1 122.5617
a2 123.6452
a3 114.7730


-----Cut Here-----
# RHF/3-21G

Dooh CO2

0 1
X1
C1 X1 1.
O1 C1 r1 X1 90.
O2 C1 r1 X1 90. O1 180.

r1 1.1558


-----Cut Here-----
# RHF/3-21G

Coov FH

0 1
F1
H1 F1 r1

hf1 0.9375


-----Cut Here-----
# RHF/3-21G

C3v CH3F

0 1
F1
C1 F1 r1
H1 C1 r2 F1 a1
H2 C1 r2 F1 a1 H1  120.
H3 C1 r2 F1 a1 H1 -120.

r1   1.4040
r2   1.0795
a1 109.3911


-----Cut Here-----
# RHF/3-21G

Dooh F2

0 1
O1
O2 O1 r1

r1 1.4025


-----Cut Here-----
# RHF/3-21G

C2v CH2F2

0 1
X1
C1 X1 1.
F1 C1 r1 X1 a1
F2 C1 r1 X1 a1 F1 180.
H1 C1 r2 X1 a2 F1  90.
H2 C1 r2 X1 a2 F1 -90.

r1   1.3724
r2   1.0731
a1  54.4480
a2 124.0684


-----Cut Here-----
# RHF/3-21G

C3v CHF3

0 1
H1
C1 H1 r1
F1 C1 r2 H1 a1
F2 C1 r2 H1 a1 F1  120.
F3 C1 r2 H1 a1 F1 -120.

r1   1.0662
r2   1.3451
a1 110.6185


-----Cut Here-----
# RHF/3-21G

Td CF4

0 1
C1
F1 C1 r1
F2 C1 r1 F1 109.4712
F3 C1 r1 F1 109.4712 F2  120.
F4 C1 r1 F1 109.4712 F2 -120.

r1 1.325


-----Cut Here-----
# RHF/3-21G

Coov NaH

0 1
Na1
H1  Na1 r1

r1 1.9266


-----Cut Here-----
# RHF/3-21G

Dooh MgH2

0 1
X1
Mg1 X1  1.
H1  Mg1 r1 X1 90.
H2  Mg1 r1 X1 90. H1 180.

r1 1.7261


-----Cut Here-----
# RHF/3-21G

D3h AlH3

0 1
Al1
H1 Al1 r1
H2 Al1 r1 H1 120.
H3 Al1 r1 H2 120. H1 180.

r1 1.5994


-----Cut Here-----
# RHF/3-21G

Td SiH4

0 1
Si1
H1 Si1 r1
H2 Si1 r1 H1 109.4712
H3 Si1 r1 H1 109.4712 H2  120.
H4 Si1 r1 H1 109.4712 H2 -120.

r1 1.4866


-----Cut Here-----
# RHF/3-21G

C3v PH3

0 1
X1
P1 X1 1.
H1 P1 r1 X1 a1
H2 P1 r1 X1 a1 H1  120.
H3 P1 r1 X1 a1 H1 -120.

r1   1.4227
a1 120.8418


-----Cut Here-----
# RHF/3-21G

C2v SH2

0 1
S1
H1 S1 r1
H2 S1 r1 H1 a1

r1  1.3505
a1 95.8054


-----Cut Here-----
# RHF/3-21G

Coov ClH

0 1
Cl1
H1  Cl1 r1

r1 1.2935


-----Cut Here-----
# RHF/3-21G

Coov BrH

0 1
Br1
H1  Br1 r1

r1 1.4326


-----Cut Here-----
# RHF/3-21G

Coov IH

0 1
I1
H1 I1 r1

r1 1.6385


-----Cut Here-----
# RHF/3-21G

Cs CH3NO2

0 1
C1
N1 C1 r1
H1 C1 r2 N1 a1
H2 C1 r3 N1 a1 H1  d1
H3 C1 r3 N1 a1 H1 -d1
O1 N1 r4 C1 a2 H1  d2
O2 N1 r4 C1 a2 H1 -d2

r1   1.4928
r2   1.0795
r3   1.0743
r4   1.2402
a1 107.0223
a2 116.8778
d1 119.2261
d2  88.8732


-----Cut Here-----
# RHF/3-21G

D6h C6H6

0 1
C1
C2 C1 r1
C3 C2 r1 C1 120.
C4 C3 r1 C2 120. C1   0.
C5 C4 r1 C3 120. C2   0.
C6 C5 r1 C4 120. C3   0.
H1 C1 r2 C2 120. C3 180.
H2 C2 r2 C3 120. C4 180.
H3 C3 r2 C4 120. C5 180.
H4 C4 r2 C5 120. C6 180.
H5 C5 r2 C6 120. C1 180.
H6 C6 r2 C1 120. C2 180.

r1 1.3846
r2 1.0721


-----Cut Here-----
# RHF/3-21G

C2v C6H5F

0 1
X1
C1 X1 r1
C4 X1 r1 C1  60.
C2 C1 r2 C4   a1 X1  90.
C3 C4 r3 C1   a2 X1 -90.
C5 C4 r3 C1   a2 X1  90.
C6 C1 r2 C4   a1 X1 -90.
F1 C1 r4 X1 120. C4 180.
H2 C2 r6 C1   a3 C6 180.
H3 C3 r7 C4   a4 C5 180.
H4 C4 r5 X1 120. C1 180.
H5 C5 r7 C4   a4 C3 180.
H6 C6 r6 C1   a3 C2 180.

r1   2.7475
r2   1.3741
r3   1.3847
r4   1.3587
r5   1.0712
r6   1.0695
r7   1.0713
a1  60.8497
a2  59.8573
a3 119.4057
a4 120.1070


-----Cut Here-----
# RHF/3-21G

D2h (para) C6H4F2

0 1
X1
C1 X1 r1
C4 X1 r1 C1 60.0
C2 C1 r2 C4   a1 X1  90.
C3 C4 r2 C1   a1 X1 -90.
C5 C4 r2 C1   a1 X1  90.
C6 C1 r2 C4   a1 X1 -90.
F1 C1 r3 X1 120. C4 180.
H2 C2 r4 C1   a2 C6 180.
H3 C3 r4 C4   a2 C5 180.
F4 C4 r3 X1 120. C1 180.
H5 C5 r4 C4   a2 C3 180.
H6 C6 r4 C1   a2 C2 180.

r1   2.7276
r2   1.3745
r3   1.3578
r4   1.0690
a1  60.6679
a2 119.5053


-----Cut Here-----
# RHF/3-21G

C2v (meta) C6H4F2

0 1
X1
C1 X1 r1
C4 X1 r1 C1  60.
C2 C1 r2 C4   a1 X1  90.
C3 C4 r3 C1   a2 X1 -90.
C5 C4 r3 C1   a2 X1  90.
C6 C1 r2 C4   a1 X1 -90.
H1 C1 r4 X1 120. C4 180.
F2 C2 r6 C1   a3 C6 180.
H3 C3 r7 C4   a4 C5 180.
H4 C4 r5 X1 120. C1 180.
H5 C5 r7 C4   a4 C3 180.
F6 C6 r6 C1   a3 C2 180.

r1   2.7633
r2   1.3731
r3   1.3829
r4   1.0673
r5   1.0706
r6   1.3545
r7   1.0687
a1  59.1478
a2  60.3254
a3 118.8018
a4 121.6725


-----Cut Here-----
# RHF/3-21G

C2v (ortho) C6H4F2

0 1
C1
C2 C1 r1
C3 C2 r2 C1 a1
C4 C3 r3 C2 a2 C1   0.
C6 C1 r2 C2 a1 C3   0.
C5 C6 r3 C1 a2 C2   0.
F1 C1 r4 C2 a3 C3 180.
F2 C2 r4 C1 a3 C6 180.
H3 C3 r5 C2 a4 C1 180.
H4 C4 r6 C3 a5 C2 180.
H5 C5 r6 C6 a5 C1 180.
H6 C6 r1 C1 a4 C2 180.

r1   1.2925
r2   1.3865
r3   1.3697
r4   1.3577
r5   1.0694
r6   1.0699
a1 121.0162
a2 120.0784
a3 120.2320
a4 117.8839
a5 120.5656


-----Cut Here-----
# RHF/3-21G

Cs C6H3F3

0 1
C1
C2 C1 r1
C3 C2 r2  C1 a1
C4 C3 r3  C2 a2  C1   0.
C5 C4 r4  C3 a3  C2   0.
C6 C5 r5  C4 a4  C3   0.
F1 C1 r6  C2 a5  C3 180.
F2 C2 r7  C3 a6  C4 180.
H3 C3 r8  C4 a7  C5 180.
F4 C4 r9  C5 a8  C6 180.
H5 C5 r10 C6 a9  C1 180.
H6 C6 r11 C1 a10 C2 180.

r1    1.3749
r2    1.3696
r3    1.3745
r4    1.3728
r5    1.3825
r6    1.3537
r7    1.3504
r8    1.0673
r9    1.3538
r10   1.0683
r11   1.0689
a1  120.5441
a2  119.0015
a3  121.3610
a4  118.9105
a5  119.5234
a6  119.9060
a7  120.7900
a8  119.6953
a9  121.2759
a10 118.8179


-----Cut Here-----
# RHF/3-21G

C3v C6H3F3

0 1
C1
C3 C1 r1
C5 C3 r1 C1  60.
C2 C5 r2 C1  30. C3   0.
C4 C1 r2 C3  30. C5   0.
C6 C3 r2 C5  30. C1   0.
F1 C1 r3 C3 150. C5 180.
H2 C2 r4 C4 150. C6 180.
F3 C3 r3 C5 150. C1 180.
H4 C4 r4 C6 150. C2 180.
F5 C5 r3 C1 150. C3 180.
H6 C6 r4 C2 150. C4 180.

r1 2.3543
r2 2.7464
r3 1.3504
r4 1.0667