[Wien] total energy when using -orbc for constant V-matrix

[Hua Wu]

Dear Stefaan Cottenier and Laurence Marks,

  Thanks a lot for your prompt replies and the useful information.

 May I understand your points as follows:  The U-related part of total energy 
is not taken into account when using -orbc, since -orbc fixes the orbital 
potential and thus the calculations skip the nomal 'x -orb -up/dn' where the 
U-related part of total energy is calculated.

 My further question is: can I regard the 'converged' total energy and forces 
calculated using -orbc as a quasi LDA/GGA result WITHIN the constrained 
orbital polarization?  If so, I may still be able to do structural 
relaxations using -orbc, e.g, to see (maybe roughly) how a lattice responds 
to the constrained orbital states (which can not be stabilized when 
using -orb).

best regards -- H. Wu

 
On Friday 15 May 2009 15:56, Laurence Marks wrote:
> I'm fairly certain that with -orbc you are not including the
> double-counting corrections which are normally calculated with x orb
> -up/dn, hence large apparent changes.
>
> On 5/15/09, Stefaan Cottenier <Stefaan.Cottenier at ugent.be> wrote:
> >  >   I tried to use LDA+U with -orbc flag to stabilize various orbital
> >  > polarized states in my calculations for some transition-metal oxides.
> >  > In order to compare their total energies, I made a simple test,
> >  > namely, run 'LDA+U with -orbc' for a well converged 'LDA+U with -orb'
> >  > case. The total energies are expected to be the same. However,  it is
> >  > surprising that in my current calculations using the latest version,
> >  > the total energies change by several Rydbergs. I am wondering if any
> >  > term is missing in the calculations of the total energy when using
> >  > -orbc. In addition, can one use -orbc to do structural relaxation like
> >  > doing LDA+U with -orb ?
> >
> > -orbc is not meant for 'final' calculations, but rather for bringing the
> >  scf cycle close to a type of solution you want. Then you turn off -orbc
> >  (i.e. use plain -orb), after which the scf cycle will spontaneously
> >  stabilize in the nearest local energy minimum, which can (but not always
> >  should) have the qualitative properties of the solution you wanted to
> >  enforce.
> >
> >  I did notice indeed that when used that way, there are several Ry's of
> >  difference between energies with and without -orbc. I cannot give you
> >  the technical explanation for this (others ?), but never cared about it
> >  as I always use the energies with plain -orb anyway.
> >
> >  Note: -orbc freezes the orbital potential that is consistent with the
> >  initial density matrices. It does not freeze the density matrices
> >  themselves: they can evolve during the scf cycle. Hence, when using
> >  -orbc as described above, you better check at the end of the -orbc scf
> >  cycle whether the final density matrices are not too different from your
> >  initial ones. If they are very different, you probably did not end up
> >  close the type of solution you wanted to enforce, and the subsequent
> >  -orb scf cycle might result into a selfconsistent solution that you do
> >  not want.
> >
> >
> >  Stefaan
> >
> >
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