[ADF-LIST] Regarding dipole moment calculation of a charged molecule

Witold Piskorz wpiskorz at chemia.uj.edu.pl
Sun Feb 11 13:37:56 CET 2018

W dniu 2018-02-11 11:36, veerapandian napisał(a):
> Dear ADF users,
>
> I am calculating dipole moment for a cluster (figure attached, four
> molecules  bound to Li+) with positive charge.
>
> After optimization, I see this "Since the molecule is NOT NEUTRAL, 
> the
> dipole moment depends on the choice of origin"  after dipole value in
> the output file.
>
> My question is, 
> How the choice of origin is defined in ADF? 
> Does ADF use center of mass in default for dipole calculation?
> Can we define the choice of origin in ADF?
>
> Any explanations would be great.
>
> Thank you in advance
>
> Best regards,
>
> Veerapandian Ponnuchamy

Hi,

I will answer in general way, independent on the piece of software you 
use.

The dipole moment of *neutral* system does not depend on the coordinate 
system origin, i.e., wherever you attach your coordinate system, you 
will obtain the same value of the dipole moment. It does not hold if the 
system is *charged*.

Namely, if you calculate the dipole moment you sum up the charges 
multiplied by the position vector. The position vector obviously depend 
on the origin. If the system is neutral, then the resultant sum *does 
not* depend on the origin. For charged it does, however.

If you want to calculate the d.m., you can use the (0,0,0) as the 
origin (and, probably, most of the codes do so), or you can use the 
centre of mass, centre of charge, whatever - for neutral system, of 
course.

I hope this informal explanation is clear :-)

With best regards,

Witold


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