Strange behaviour by molecule in Electric field

[Matt W]

Hi,

You make a very good point! Now for your argument to hold your system needs 
to be translationally invariant. There are two reasons (at least) why it is 
not.

(i) the EFIELD is non periodic (i.e. a saw tooth), so there is a huge field 
at the cell edge. You need to centre your molecule in the box.
(ii) your CUTOFF is not truly sufficient if you really want to see the 
molecule stay centred - there's a post from Flo discussing this concerning 
vibrational analysis from a month or so ago, but this is a minor point.

BUT, I think you have also identified a bug. According to me, there is an 
inconsistency between the way forces due to the field act on atoms cores 
and electrons.

changing line 233 (or thereabouts) of efield_utils.F to

force(ikind)%efield(:,iatom)=force(ikind)%efield(:,iatom)-field*zeff

i.e. changing the sign of the forces on the cores, seems to give good 
conservation of force in my quick tests (plus comments above). You'll have 
to recompile, of course.

Let us know if this seems to work for you (it is possible that the force on 
the electrons rather than the core should be inverted, but at the moment 
they are both being pushed in the same direction).

Matt

On Friday, 8 November 2013 09:10:51 UTC, Pankaj Mishra wrote:
>
> Hello,
>
> As per my fundamental knowledge, polar molecule can rotate in electric 
> field, not translate. Being more precise, here i am talking about 1 
> molecule in non-periodic box. I didn't find any force to move in the field. 
>
> Even non-polar molecule and single atom (H and Ne) are also translating in 
> EF in CP2K. In the case of water cluster, the center of mass of whole 
> system is translating. I have not found such behavior in other package like 
> GAMESS .
>
> This is a very serious issue for me. Please try to suggest something if i 
> am missing anything in my input.
>
> Thank you
>
> Pankaj 
>
> On Thursday, 7 November 2013 20:23:58 UTC+1, Matt W wrote:
>>
>> Hi,
>>
>> just wondering: why would you not expect a polar molecule to move in an 
>> electric field?
>>
>> Matt
>>
>
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