TIP3P

TIP3P, or Transferable Intermolecular Potential with 3 Points, is a widely used rigid, non-polarizable water model in molecular dynamics. It represents a water molecule with three interaction sites: a Lennard-Jones center on the oxygen atom and two fixed point charges located on the hydrogen atoms.

The geometry is fixed with an O–H bond length of 0.957 Å and a H–O–H angle of 104.45 degrees. Partial charges are q_H = +0.417 e and q_O = −0.834 e, ensuring overall neutrality. Lennard-Jones interactions are centered on the oxygen, while the hydrogen sites carry no LJ parameters. Electrostatic interactions are treated with standard Coulombic terms.

TIP3P was introduced by Jorgensen, Chandrasekhar, Madura, and Klein in 1983 as one of the first simple, transferable water models for computer simulations. It is designed to be computationally efficient and is compatible with common molecular dynamics packages and long-range electrostatics methods such as Ewald summation or Particle Mesh Ewald (PME).

It became a popular choice for biomolecular simulations because of its low cost and reasonable accuracy for many properties. While TIP3P offers speed and simplicity, it has limitations. As a rigid, non-polarizable model, it cannot capture electronic polarization. Some thermodynamic and dynamic properties, such as the dielectric constant, diffusion, and interfacial behavior, are not as accurately reproduced as with more advanced models like TIP4P/2005. Despite this, TIP3P remains widely used, often in tandem with TIP3P-Ewald variants optimized for long-range electrostatics.