With the key OCCUPATIONS you can specify in detail the assignment of electrons to MOs
OCCUPATIONS Options
{irrep orbitalnumbers
irrep orbitalnumbers
...
End }
Occupations
is a general key: it has an argument or a data block. If you want to use both, the continuation code ( &) must be appended at the end of the argument.
Options
May contain Keeporbitals, Smearq, Freeze, or Steep:
Keeporbitals=NKeep
Until SCF cycle Nkeep electrons are assigned to MOs
according to the Aufbau principle, using at each cycle the then current orbital
energies of the MOs. Thereafter the KeepOrbitals feature is applied. As soon as this is activated
the program will on successive SCF cycles assign electrons to the MOs that
maximally resemble - in spatial form - those that were occupied in a 'reference
cycle number'. The default for Nkeep is
20, except:
a) When orbital occupations for MOs are specified explicitly in the data block
of the occupations key, these apply throughout.
b) In a Create run fixed occupations are derived from a database in the program.
c) When electron smearing is explicitly turned on by the user
(see the Smearq option below) Nkeep is by default 1,000,000 so
the program will 'never' compare the spatial forms of MOs to determine the
occupation numbers.
The 'reference cycle number' is by default the previous cycle, which will
suppress jumps in the spatial occupations during the SCF development while at
the other hand allowing the system to let the more-or-less-frozen configuration
relax to self-consistency.
Freeze
Occurrence of this word in the option list specifies that the 'reference cycle number' will be the cycle number on which the KeepOrbitals feature is activated: during all subsequent SCF cycles the program will assign electrons to MOs that resemble the MOs of that specific SCF cycle. This may be used when the MOs of that cycle are already reasonably close to the final ones, and it will suppress unwanted step-by-step charge-transfers from occupied to empty orbitals that are very close in energy. By default this option is not active.
Smearq=Smear1[,Smear2,Smear3,...,Smear10]
SmearN is half the energy width (in hartrees) over which electrons
are smeared out over orbitals that lie around the fermi level and that are
close in energy. Smearing is a trick that may
help when the SCF has problems converging.
One should be well aware that the physical meaning of a result obtained with
smeared occupations is unclear (to express it mildly). It may be useful to get
over a hurdle in a geometry optimization.
By default the initial smear parameter
is zero (i.e.: smearing is not applied). It is turned on automatically by the
program when SCF convergence is
found to be problematic, but only in an optimization-type application (simple
optimization, linear transit, transition state) when the geometry is not
yet converged.
You can rigorously prohibit any smearing by specifying it explicitly with value
zero. More generally: specifying the smear parameter makes the program to apply
it always, but always with the input-specified value.
When a comma-delimited list of values is specified, after SCF has converged,
the next value from the list is picked and the SCF is continued.
This way one can specify a list of gradually decreasing values
to get sort of annealing effect. NOTE: No spaces are allowed when specifying a
list of values for Smearq.
Steep=Lambda[,Nmax]
The occupation number for each orbitals are updated according to steepest-descent
method (Ref: F. W. Averill and G. S. Painter, Phys. Rev. B 46, 2498 (1992)).
During an SCF cycle, the occupation number for each new orbital is initially
determined by decomposing the old charge density with new orbitals. Then, the
occupation numbers are modified so that the total energy of the system will decrease.
The Lambda parameter gives the coefficient for the charge transfer in 1/au unit.
The second parameter, Nmax, is an additional limit for the amount of the
charge transfer. Nmax would be useful for early steps of cycle when the Lambda
parameter gives too large charge transfer. Too small Nmax results in irregular
behavior in SCF convergence. In the case of difficult SCF convergence,
you should make mixing and Lambda smaller. From our experience, Nmax=0.1 or 0.2
is usually OK.
This method should be used with turning off DIIS method (DIIS N=0), and the choice
of the mixing parameter in SCF cycle is also important. This option is especially
useful for systems with many quasi-degenerate orbitals around Fermi level.
For instance, cluster models of surface systems usually suffer from dangling bonds
and should be converged with this method. Note though that slow convergence is an
intrinsic feature of this method so one should specify a large limit for the number
of SCF cycles, say 500 or even 1000, depending on the cluster size.
irrep
The name of one of the irreducible representations (not a subspecies) of the point group of the system. See the Appendix for the irrep names as they are used in ADF.
orbitalnumbers
A series of one or more numbers: the occupation numbers for one-electron valence orbitals in that irrep. The orbitals are ordered according to their energy eigenvalue; higher states than those listed get an occupation number zero.
For degenerate representations such as the 2-dimensional E-representations or the 3-dimensional T-representations, you must give the total occupation, i.e. the sum over the partner representations; adf assigns each partner an occupation equal to the appropriate fraction of what appears here.
In an unrestricted calculation, two sequences of numbers must be specified for each irrep; the sequences are separated by a double slash (//). The first set of numbers is assigned to the spin-α orbitals, the second set to the spin-β orbitals. Note that this is not meaningful in an unrestricted Spin-Orbit coupled calculation using the (non-)collinear approximation, where one should use one sequence of occupation numbers for each irrep.
Notes about the occupations data block:
Notes about the occupations options:
