### pSPICA Force Field

Tutorial for pSPICA FF
pSPICA is a CG FF designed with a polar CG water model. The CG modeling method of pSPICA is the same as the SPICA FF. Details of the SPICA FF can be found

here. The full details of pSPICA can be found in the publication

here. The following picture shows the polar CG water model.

The polar CG water composed of a pair of interacting sites, WO and WH, is designed to represent three neighboring water molecules in aqueous solution. The two sites, WO and WH, are connected by a rigid bond and carry positive and negative partial charges, respectively. Nonbonded interaction is described by a Lennard-Jones (LJ) interaction and the electrostatic interaction.
For the LJ interaction, the following LJ12-5 function type was chosen:

where

*ε*_{W} and

*σ*_{W} are LJ parameters. The prefactor of the LJ12-5 function is decided to have the minimum value as -

*ε*_{W} and to attain zero at

*r*_{ij} =

*σ*_{W}. The LJ12-5 function is adopted to reproduce the experimental density, surface tension, and dielectric permittivity of water. The LJ interaction is considered only among WO sites.

For electrostatic interactions in pSPICA, the relative dielectric constant (

*ε*_{r}) = 3.2 is used.

In the SPICA FF,

*ε*_{r} = 80 is chosen to compensate for the exclusion of dipoles from the water model, assuming that the polar CG segments are always fully hydrated and the electrostatic interaction is fully screened.

Details of bonded interactions and other nonbonded interactions in pSPICA are the same as the SPICA FF.