Difference in reported dipole moment between `IDIPOL=1` and `IDIPOL=4` at zero electric field (VASP 6.2.0)

[rajaraman]

Dear VASP developers,

I am performing non-collinear DFT calculations on an isolated molecular system (23 × 23 × 23 Å cubic cell) using VASP 6.2.0. The molecule is placed in a large vacuum region, and the geometry, POTCAR, INCAR (except for `IDIPOL`), KPOINTS, and all other settings are identical between the calculations.

At **zero electric field** (`EFIELD` not specified), I obtain:

**Case 1**

```
LDIPOL = .TRUE.
IDIPOL = 4
```

Output:

```
dipole moment = (0.044124, -0.046093, -0.000363) eÅ
```

**Case 2**

```
LDIPOL = .TRUE.
IDIPOL = 1
```

Output:

```
dipole moment = (0.127493, 0.000000, 0.000000) eÅ
```

Everything else is identical, including:

* same geometry
* same magnetic configuration
* same cell
* same convergence settings
* same number of electrons
* no external electric field

My expectation was that, at zero field, changing `IDIPOL` should only modify the dipole correction procedure and should not significantly alter the computed molecular dipole moment. In particular, I expected the x-component to remain nearly unchanged and the y and z components to still be reported.

Could you please clarify:

1. Does `IDIPOL=1` intentionally report only the dipole component along the correction direction?
2. Is the `dipolmoment` printed in OUTCAR fundamentally different for `IDIPOL=1` and `IDIPOL=4`?
3. If one wants to compare the full molecular dipole vector before and after applying an electric field along x, which setup is recommended?

Thank you very much for your help.

[michael_wolloch]

Dear Rajaraman,

IDIPOL=4 is the correct value for isolated molecules, so you should use that if you want to compare dipole moments with and without an electric field. However, fundamentally, I would also expect that the dipole moments in the x direction should be the same for both of your calculations, since the computation for all directions is the same.

Please upload zipped folders with your input and output files, as per the posting guidelines, so I can analyze properly what is going on in your system.

Cheers, Michael