Dipole moment from Berry phase

Ali rnalo... at gmail.com
Fri Jan 6 10:30:28 CET 2012


Dear CP2K Users,

I have been trying to use the Berry Phase formalism to compute the
dipole moment of some crystal systems with non-standard unit cells
(rhombohedral) but have been getting some strange results so I
resorted to doing some benchmarks for a just a single water molecule
in an orthorhombic cell of side length 9.5Angstroms - the results
still seem rather strange. I am hoping that someone can set me
straight on my interpretations:

* Single Water in 9.5, 90 90 90, the dipole moment is reasonable:
2.18943 Debye (using MULTIPLE_UNIT_CELL 1 1 1)

* I then do several combinations of expanding the cell in different
directions: MULTIPLE_UNIT_CELL 2 1 1, 1 2 1, and 1 1 2 and I also get
reasonable values for the total dipole moment of the supercell:
4.37213,
4.27032 and 4.26632 Debye respectively. Thus the dipole moment per
unit cell (divided by 2) is also reasonable.

* When I then expand to MULTIPLE_UNIT_CELL 2 2 2 and 3 3 3 I get
results that don't quite make sense to me:

2 2 2: 38.55847 Debye and 3 3 3: 55.44474 Debye. Thus the dipole
moment per unit cell in 2 2 2 is 4.81 Debye and in the 3 3 3 its
2.06841 Debye. These results don't make sense to me. Shouldn't the
dipole moment per unit cell be close to 2.18 as in the 2 1 1, 1 2 1
and 1 1 2 systems?

Any help/pointers on this will be appreciated.

Thanks in advance.

Ali


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