Calculator Input
Enter a probe point and up to six atomic sites. Blank rows are ignored automatically.
Example Data Table
This example uses common water partial charges with a probe placed 1.500 Å above oxygen.
| Atom | Charge (e) | x (Å) | y (Å) | z (Å) | Probe Distance (Å) | Approx. Contribution (V) |
|---|---|---|---|---|---|---|
| O | -0.834 | 0.0000 | 0.0000 | 0.0000 | 1.5000 | -8.006 |
| H1 | 0.417 | 0.9572 | 0.0000 | 0.0000 | 1.7794 | 3.375 |
| H2 | 0.417 | -0.2390 | 0.9270 | 0.0000 | 1.7789 | 3.376 |
| Estimated Total Potential | -1.255 V | |||||
Formula Used
The exact molecular electrostatic potential is obtained from nuclei and electron density. In quantum chemistry, the continuous form is:
This calculator uses an advanced point-charge approximation based on user-supplied partial charges and coordinates:
Where ke is Coulomb’s constant, εr is the relative dielectric constant, qi is the atomic partial charge in elementary-charge units, and ri is the distance between the probe and each atomic site.
The electric field vector is also estimated:
Potential is additionally converted to kJ/mol and kcal/mol per unit charge for chemically useful comparison.
How to Use This Calculator
- Select a distance unit that matches your coordinate set.
- Enter the relative dielectric constant for vacuum, solvent, or medium screening.
- Set the probe point where you want the electrostatic potential evaluated.
- Fill atomic labels, partial charges, and Cartesian coordinates for each site.
- Use a small distance floor to avoid singularities when the probe is extremely close to an atom.
- Submit the form to view total potential, field magnitude, per-atom contributions, and the Plotly graph.
- Download the contribution table as CSV or save the result section as PDF.
Frequently Asked Questions
1) What does a positive molecular electrostatic potential mean?
A positive value suggests the selected region is relatively electron-deficient in this approximation. Such regions often attract electron-rich species and may align with electrophilic behavior in qualitative comparisons.
2) What does a negative molecular electrostatic potential mean?
A negative value suggests the chosen probe point lies in a relatively electron-rich region. These areas often attract positively charged or electron-poor probes in qualitative reactivity analysis.
3) Is this the same as an ab initio electrostatic potential map?
No. This tool estimates potential from fixed point charges. Ab initio maps use full electron density and nuclear terms from quantum calculations, so they are more rigorous and spatially detailed.
4) Why does dielectric constant change the result?
The dielectric constant models environmental screening. Higher values reduce electrostatic interactions, so the calculated potential and field magnitude become smaller than vacuum values.
5) Why is a distance floor included?
Electrostatic point-charge equations diverge when distance approaches zero. The floor prevents unrealistic spikes and keeps the estimate numerically stable when the probe nearly overlaps an atomic site.
6) Which partial charges should I use?
Use charges from the same model and geometry set whenever possible. RESP, CHELPG, Mulliken, and NPA values can differ, so consistent charge assignment matters for meaningful comparisons.
7) Can I compare two molecules with this calculator?
Yes, if you use comparable geometries, units, probe locations, dielectric assumptions, and charge methods. Consistency is essential for interpreting relative differences in predicted electrostatic behavior.
8) What does the Plotly graph show?
The graph plots each atomic contribution to the probe potential and overlays the cumulative potential after each added atom. It helps identify dominant sites and the build-up of the total estimate.