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IMIX = 0 | 1 | 2 | 4
Default: IMIX = 4 

Description: IMIX specifies the type of mixing.

The mixed density is given by
with =AMIX and =BMIX
If BMIX is chosen to be very small, e.g. BMIX=0.0001, a simple straight mixing is obtained. Please mind, that BMIX=0 might cause floating point exceptions on some platforms.
  • IMIX=2: A variant of the popular Tchebycheff mixing scheme.[2]
In our implementation a second order equation of motion is used, that reads:
with =AMIX, =BMIX, and =AMIN.
A simple velocity Verlet algorithm is used to integrate this equation, and the discretized equation reads (the index N now refers to the electronic iteration, F is the force acting on the charge):
For BMIX≈0, no model for the dielectric matrix is used. It is easy to see, that for a simple straight mixing is obtained. Therefore, corresponds to maximal damping, and obviously implies no damping. Optimal parameters for and AMIX can be determined by converging first with the Pulay mixer (IMIX=4) to the groundstate. Then the eigenvalues of the charge dielectric matrix as given in the OUTCAR file must be inspected. Search for the last orrurance of
eigenvalues of (default mixing * dielectric matrix)
in the OUTCAR file. The optimal parameters are then given by:
  • IMIX=4: Broyden's 2nd method,[3][4] or Pulay's mixing method[5] (depending on the choice of WC).
With the default settings, a Pulay mixer with an initial approximation for the charge dielectric function according to Kerker[1] is used
where =AMIX, =BMIX, and =AMIN.
A reasonable choice for AMIN is usually AMIN=0.4. AMIX depends very much on the system, for metals this parameter usually has to be rather small, e.g. AMIX= 0.02.
In the Broyden scheme, the functional form of the initial mixing matrix is determined by AMIX and BMIX (or alternatively specified by means of the INIMIX-tag). The metric used in the Broyden scheme is specified through MIXPRE.

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