# I generated Wannier centers but am having trouble understanding how they relate to the atoms in my system: there are more Wannier centers than atoms?

I'm trying to calculate dipole moments from Wannier centers for the first time and there are basics I don't understand and can't figure out from online resources.

The software I'm using has a simple "output Wannier centers" option. So I now have output, which is a file with Wannier centers and atomic positions. My system contains many water molecules but here's a simple example for a system that has only one H2O. The "X" are Wannier centers:

X          6.50000026       6.18706578       6.50000000
X          6.50000000       6.71337672       6.50000002
X          6.50000008       6.95339960       6.49999998
X          6.49999971       6.54636884       6.50000000
H          6.50000000       7.09480000       7.26880000
H          6.50000000       7.09480000       5.73120000
O          6.50000000       6.50000000       6.50000000

• Why are there four X for one H2O? I vaguely assumed that there would be one X, corresponding to one H2O. Do the four X correspond somehow to different molecular configuration possibilities? I have tried to look this up but probably don't even know the right search terms.

I included "Wannier90" as a tag because it seems relevant and generates output formatted like my example but I am not using Wannier90.

Determining localized orbitals [1] (for periodic boundary conditions these are called Wannier functions), the 4 orbitals localize into two covalent OH bonds, and two lone pairs on the oxygen atom. You have obtained the centroids of these orbitals, which are determined by the expectation value $$\langle {\bf r} \rangle$$.