---
title: Electrostatic Potential (MEP) plugin
url: https://www.moldraw.com/docs/plugins/mep-plugin-molecular-electrostatic-potential-surface/
section: Plugins
updated: 2026-10-11
publisher: LabCanvas
---

# Electrostatic Potential (MEP) plugin

> Install MEP from Settings, Plugins, draw a molecule, open the MEP tab and press Show in 3D.

![MolDraw MEP plugin with a red-to-blue electrostatic potential scale and a table of Gasteiger partial charges per atom](https://www.moldraw.com/docs/img/plugin-mep-annotated-screenshot.webp)
*MEP for chloramphenicol. Oxygen atoms are most negative (red). Carbons with H are a bit positive (blue).*

1. MEP tab in the analysis panel.
2. Colour scale from most electron-rich (red) to electron-poor (blue).
3. Show in 3D draws the coloured surface on the 3D model; Refresh charges recalculates after edits.
4. Opacity slider for the surface.
5. Charge table: Gasteiger partial charge for every heavy atom, sorted most negative first.

## What it does

The plugin works out Gasteiger partial charges for each atom. Then it colors a surface around the 3D model. Red is negative. Blue is positive.

It is fast, but it is not a quantum calculation. Use it to compare, not for exact energies. For example, see which end is more negative or where a nucleophile will attack.

## Who it is for

Students who learn polarity. Teachers who make slides about charge. Anyone who wants a quick picture of charge in a molecule.

## How to use it

1. Open the [MolDraw editor](https://www.moldraw.com/) and sign in. Click **Settings**, then **Plugins**, then **Install** next to Electrostatic Potential (MEP).
2. Draw a molecule (or load one by name using the PubChem search box).
3. Open the **MEP** tab (callout 1); charges are calculated immediately.
4. Read the colour scale (callout 2) and the charge table (callout 5).
5. Press **Show in 3D** (callout 3) to paint the surface on the 3D model.
6. Adjust **Opacity** (callout 4) to see the ball-and-stick model through the surface.
7. Use **Turn surface off** to return to the plain 3D view.

## Tips

- Compare related molecules, e.g. phenol vs. aniline, to see how substituents change charge.
- After editing the 2D drawing, press Refresh charges.
- For bond polarity, see the [Lewis structure generator](https://www.moldraw.com/tools/free-chem-tools/lewis-structure-generator.html) and [VSEPR calculator](https://www.moldraw.com/tools/free-chem-tools/vsepr-molecular-geometry-calculator.html).

## Questions and answers

**What does an electrostatic potential map show?**
How positive or negative the electric potential is at the surface of a molecule. Red regions attract positive charges (lone pairs, electronegative atoms); blue regions are electron-poor.

**How is the MEP calculated in MolDraw?**
From Gasteiger partial atomic charges mapped onto a solvent-accessible surface. It is an approximation, not a quantum-chemical ESP.

**Is it free?**
Yes, with a free MolDraw account.