Mixing

Name MixField
Section SCF::Mixing
Type integer

Selects what should be mixed during the SCF cycle. Note that currently the exact-exchange part of hybrid functionals is not mixed at all, which would require wavefunction-mixing, not yet implemented. This may lead to instabilities in the SCF cycle, so starting from a converged LDA/GGA calculation is recommended for hybrid functionals. The default depends on the TheoryLevel and the exchange-correlation potential used.

Options:

• none:
  No mixing is done. This is the default for independent particles.
  
• potential:
  The Kohn-Sham potential is mixed. This is the default for other cases.
  
• density:
  Mix the density.
  
• states:
  (Experimental) Mix the states. In this case, the mixing is always linear.
  

Name Mixing
Section SCF::Mixing
Type float
Default 0.3

The linear, Broyden and DIIS scheme depend on a "mixing parameter", set by this variable. Must be 0 < Mixing <= 1.

Name MixingPreconditioner
Section SCF::Mixing
Type logical
Default false

(Experimental) If set to yes, Octopus will use a preconditioner for the mixing operator. This preconditioner is disabled for systems with dimension other than 3.

Name MixingResidual
Section SCF::Mixing
Type float
Default 0.05

In the DIIS mixing it is benefitial to include a bit of residual into the mixing. This parameter controls this amount.

Name MixingRestart
Section SCF::Mixing
Type integer
Default 20

In the Broyden and Bowler_Gillan schemes, the mixing is restarted after the number of iterations given by this variable. Set this to zero to disable restarting the mixing.

Name MixingScheme
Section SCF::Mixing
Type integer
Default broyden

The scheme used to produce, at each iteration in the self-consistent cycle that attempts to solve the Kohn-Sham equations, the input density from the value of the input and output densities of previous iterations.

Options:

• linear:
  Simple linear mixing.
  
• broyden:
  Broyden scheme [C. G Broyden, Math. Comp. 19, 577 (1965); D. D. Johnson, Phys. Rev. B 38, 12807 (1988)]. The scheme is slightly adapted, see the comments in the code. For complex functions (e.g. Sternheimer with EMEta > 0), we use the generalization with a complex dot product.
  
• diis:
  Direct inversion in the iterative subspace (diis) scheme [P. Pulay, Chem. Phys. Lett., 73, 393 (1980)] as described in [G. Kresse, and J. Hurthmueller, Phys. Rev. B 54, 11169 (1996)].
  
• bowler_gillan:
  The Guaranteed-reduction modification of the Pulay scheme by Bowler and Gillan [D. R. Bowler and M. J. Gillan, Chem. Phys. Lett. 325, 473 (2000)].
  

Name MixInterval
Section SCF::Mixing
Type integer
Default 1

When this variable is set to a value different than 1 (the default) a combined mixing scheme will be used, with MixInterval

• 1 steps of linear mixing followed by 1 step of the selected mixing. For the moment this variable only works with DIIS mixing.
  
  

Name MixNumberSteps
Section SCF::Mixing
Type integer
Default 4

In the Broyden and Bowler_Gillan schemes, the new input density or potential is constructed from the values of the densities/potentials of a given number of previous iterations. This number is set by this variable. Must be greater than 1.