TIP4P

TIP4P is a widely used four-site rigid water model employed in molecular dynamics simulations. It represents a water molecule with three atoms (two hydrogens and one oxygen) plus a fourth, massless site, known as the M site, which carries the negative charge. The negative charge on the M site is placed at a fixed distance from the oxygen along the bisector of the HOH angle, while the Lennard-Jones interaction is centered on the oxygen atom. The model is non-polarizable, meaning it does not allow for electronic polarization in response to its environment.

In TIP4P, the hydrogens carry positive partial charges, the M site carries a negative charge, and the oxygen center provides the Lennard-Jones interactions. The geometry is fixed, with a rigid bond length and bond angle that define the molecular shape. This configuration is designed to reproduce the electrostatic distribution and quadrupole moment of real water more accurately than simpler three-site models.

TIP4P was developed to improve the representation of essential water properties in simulations, including density, phase behavior, and dynamic properties, relative to earlier rigid models. It has become a standard reference for bulk water and aqueous environments and is often used in studies of solvation, biomolecular systems, and fluid phases. Variants such as TIP4P/2005 and TIP4P-Ew have been developed to optimize performance for specific simulation methods or conditions, while preserving the four-site framework.

Limitations include the lack of explicit electronic polarization and fixed geometry, which can limit accuracy in highly heterogeneous or extreme environments. Nonetheless, TIP4P remains a foundational tool in computational chemistry and materials science for modeling water.