Orientation of the electric field when simulating membrane potential

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1

GROMACS version:
GROMACS modification: No

Hi,

This is both a sanity check (please tell me if my logic is backwards) and a comment for anyone who might be trying to set up the same thing.

I am trying to implement a constant electric field in my system that would mimic the membrane potential (-70 mV in the intracellular space). If a membrane is set on an xy-plane, and for a simulation box with a Z-axis of 5 nm, I am using:
electric-field-z = 0.014 0 0 0 ; E = V/Lz = 0.07 V / 5 nm

To assess the orientation of this el. field, I ran short simulations of i) one water molecule in vacuum; and ii) NaCl in vacuum. In both cases, the component with positive (partial) charge - so, hydrogens in water or Na+ in NaCl - orients towards the +Z.

This would suggest to me that the field is implemented as -Z: positive —> +Z: negative. This would also suggest to me that, if we want it to mimic the membrane potential (which is negative in the intracellular space) the system needs to be set up upside down, with the inner components of the system in the +Z area of the box.

For the more experienced in the field - does this make sense?

2

You are missing a sign in your equation. The electric field is equal to minus the gradient of the potential.So a positive charges experiences a force in the direction of the field, as you observe.