[Wannier] Wannier90-VASP: Dipole moment of isolated H20

[Maurice de Koning]

Hello all,

We have started using the Wannier 90 interface to VASP and as a first test case we are trying to
reproduce the Wannier centers for the isolated water molecule as in the paper by Silvestrelli and Parrinello (JCP 111 3572,1999).

Using the PAW approach, the PBE xc functional, a cubic periodic box with side 10.6 Angstrom, a plane-wave kinetic energy cut-off of 1000 eV and Gamma-point sampling, the
relaxed geometry of the water molecule is identical to that reported by Silvestrelli, with an O-H distance of 0.972 Angstroms and an internal angle of 104.4 degrees.

However, when using the Wannier90 interface to VASP, the Wannier centers that we obtain do not coincide with those reported in Silvestrelli. The position of the Wannier
centers corresponding to the lone pairs are reasonable. We find distances of 0.306 Angstroms from the oxygen ion, and an internal angle of 120.8 degrees, which agrees reasonably well with the  values of 0.300 Angstrom and 126.4 degrees reported by Silvestrelli.

The WFCs associated with the electrons in the covalent bonds, however, are very different. We find a distance from the oxygen ion of 0.47 Angstrom versus a value of 0.53 Angstrom reported by Silvestrilli. These differences lead to quite a different electric dipole moment.

Any suggestions as to what might be the reason for these discrepancies?

Cheers,

Maurice