# difference between Total energy and ENERGY| Total FORCE_EVAL ( QS ) energy ?

Tue Mar 23 14:10:10 UTC 2010

```Many thanks for the reply, Prof. Hutter.
Setting multiplicity 73  means maximum spin for the system, i.e.
ferromagnetic ordering. It might be relatively simple for this
ordering.
The three sets of energies I gave are for the same geometry. They are
from single point calculations, not MD  nor geoopt. If the three
calculations found different electronic states, I am still puzzled how
come "ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.)" are almost the
same while "Total energies" are not.

Regards,
Jun

On Mar 23, 8:20 am, Juerg Hutter <hut... at pci.uzh.ch> wrote:
> Hi
>
> my guess would be that
> MULTIPLICITY 73
> is the problem. There is a high probability that you converged
> in an excited state (and there are probably an enormous
> amount of them with close energy) and when you restart
> or make an MD/geometry move you will find another state.
> The only way I can see that such a calculation is feasible
> is with non-collinear spin, a method not implemented in cp2k.
>
> regards
>
> Juerg Hutter
>
> ----------------------------------------------------------
> Juerg Hutter                   Phone : ++41 44 635 4491
> Physical Chemistry Institute   FAX   : ++41 44 635 6838
> University of Zurich           E-mail: hut... at pci.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zurich, Switzerland
> ----------------------------------------------------------
>
> On Mon, 22 Mar 2010, Jun wrote:
> > Hi all,
>
> > When doing electronic structure calculations in MD or geo_opt, it is
> > very often to see a small difference in energy between "Total
> > energy:      ****  " and " ENERGY| Total FORCE_EVAL ( QS ) energy
> > (a.u.):   **** " in output files. The difference is usually so small
> > that I thought it is just due to numerical error. (I am not saying the
> > difference due to applying some restraining potentials or in MIXED
> > calculations). However, I recently noticed that the difference can be
> > as large as 0.1 eV, which makes me wonder what is difference in
> > definitions of "Total energy " and "ENERGY| Total FORCE_EVAL ( QS )
> > energy". An example is as follows: (input file (input.inp) is
>
> > I have repeated the same ENERGY_FORCE calculation three times (there
> > may be some differences in the initial guess for wavefunction because
> > I used SCF_GUESS RESTART.), the energies I got are
> > 1.  Total energy:
> > -3269.58265002363760
> > ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
> > -3269.578462626209330
>
> > 2.   Total energy:
> > -3269.57862048247534
> > ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
> > -3269.578462728981776
>
> > 3.   Total energy:
> > -3269.57793566477721
> > ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
> > -3269.578462722795848
>
> > In three cases, " ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.)" are
> > almost the same, but  "Total energy: " are quite different. Especially
> > for case 1, the difference is up to 0.1 eV.
>
> > Any suggestion and comment are appreciated.
>
> > Cheers,
> > Jun
>
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