Confusion: cation and anion move towards the same direction under an external electric field

[Tonys_Yu]

Dear Marcella, 

I'm sorry to bother you again for my new question. I have checked the 
keyword of PERIODIC_EFIELD to apply an external electric field 0.1V/A, 
which seems work well, as shown below. The model contains 30 water 
molecules and a ion (chloride ion or potassium ion). But I found that, in 
the two independent calculations, the chloride ion and potassium ion moved 
towards the direction of electric field and the opposite direction, 
respectively, which didn't match the reality as far as I know.  Am I 
missing something important? 

      &PERIODIC_EFIELD
       INTENSITY  0.00194468915     #1V/A=0.0194468915 a.u.
       POLARISATION 1 0 0         
      &END PERIODIC_EFIELD

Tonys Yu

在 2017年7月19日星期三 UTC+8下午6:32:48，Marcella Iannuzzi写道：
>
> Hi,
>
> this external potential is not an electric field, it is just a linear 
> potential.
> Check the keywords EFIELD and PERIODIC_EFIELD.
>
> Marcella
>
> On Wednesday, July 19, 2017 at 12:20:54 PM UTC+2, Tonys_Yu wrote:
>>
>> Hello,
>>        I want to study a ionic displacement in a bulk water box with 30 
>> water molecules. External electric field is 0.1V/A, and added by 
>> EXTERNAL_POTENTIAL command as below:
>>
>>       &EXTERNAL_POTENTIAL
>>        FUNCTION (A/B)*X
>>        PARAMETERS A B
>>        VALUES [eV] 1.0 [angstrom] 10.0
>>       &END EXTERNAL_POTENTIAL
>>
>>     What makes me confused is that both water molecules and K-ion are 
>> moving along the negative direction of X. Moreover, when I changed the 
>> K-ion by a Cl-ion or reduced the electric field to a smaller value 
>> (0.001V/A), the same results was observed. It seems contrary to the fact 
>> that, with the same electric field (value and direction), K-ion and Cl-ion 
>> would move towards opposite directions. 
>>     I beg someone could shed light on my confusion. Necessary files have 
>> been attached.
>>
>> Tonys Yu
>>
>
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