DelPhi is a scientific application which calculates electrostatic potential and energies in systems of biomolecules and geometrical objects. One of the main problems in modeling the electrostatic potential of biological macromolecules is that they exist in water at a given ionic strength

Ionic strength

The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...

 and that they have an irregular shape. Analytical solutions of the corresponding Poisson-Boltzmann equation

Poisson-Boltzmann equation

The Poisson–Boltzmann equation is a differential equation that describes electrostatic interactions between molecules in ionic solutions. It is the mathematical base for the Gouy–Chapman double layer theory; first proposed by Gouy in 1910 and complemented by Chapman in 1913...

 (PBE) are not available for such cases and the distribution of the potential can be found only numerically. DelPhi, developed in Dr. Barry Honig lab in 1986, was the first PBE solver used by many researchers. The widespread popularity of DelPhi is due to its speed, accuracy (calculation of the electrostatic free energy is only slightly dependent on the resolution of the grid) and the ability to handle extremely high grid dimensions. Additional features such as assigning different dielectric constants to different regions of space, modeling geometric objects and charge distributions, and treating systems containing mixed salt solutions also attracted many researchers. In addition to the typical potential map, DelPhi can generate and output the calculated distribution of either the dielectric constant or ion concentration, providing the biomedical community with extra tools for their research.

PDB files

Protein Data Bank (file format)

The Protein Data Bank file format is a textual file format describing the three dimensional structures of molecules held in the Protein Data Bank. The pdb format accordingly provides for description and annotation of protein and nucleic acid structures including atomic coordinates, observed...

are typically used as input for DelPhi calculations. Other required inputs are an atomic radii file and a charge file. Binary Potential files as output from DelPhi can be viewed in most molecular viewers, and can either be mapped onto surfaces or visualized at a fixed cutoff.

DelPhi is commonly used in protein science to visualize variations in electrostatics along a protein or other macromolecular surface.