About OOPSE OOPSE News Features Download CVS Mailing Lists Documentation Authors Funding Results	The WATER Force Field In addition to the DUFF force field's solvent description, a separate WATER force field has been included for simulating most of the common rigid-body water models. This force field includes the simple and point-dipolar models (SSD, SSD1, SSD/E, SSD/RF, and DPD water), as well as the common charge-based models (SPC, SPC/E, TIP3P, TIP4P, and TIP5P).[17,23,16,25,26,27,28,29] In order to handle these models, charge-charge interactions were included in the force-loop: $$V_{\text{charge}}(r_{ij}) = \sum_{ij}\frac{q_iq_je^2}{r_{ij}},$$ (3.19) where $q$ represents the charge on particle $i$ or $j$ , and $e$ is the charge of an electron in Coulombs. The charge-charge interaction support is rudimentary in the current version of OOPSE. As with the other pair interactions, charges can be simulated with a pure cutoff or a reaction field. The various methods for performing the Ewald summation have not yet been included. Also, the charge-dipole and charge-quadrupole (for interactions between SSD type water and charges) are not yet available, so it is currently inadvisable to mix dipolar and charge based molecules in the same system. The WATER force field can be easily expanded through modification of the WATER force field file (WATER.frc). By adding atom types and inserting the appropriate parameters, it is possible to extend the force field to handle rigid molecules other than water.
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	Updated on January 16, 2006