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

[Tonys_Yu]

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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