"""A module for defining the TIP3P forcefield
This definition of the TIP3P forcefield includes bond and bond angle strengths
as these are needed for to create the required HarmonicPotentials. As a result,
they can be used for simulating a flexible water molecule. However, TIP3P
itself is a rigid model, and in order to replicate this a constraint algorithm
should be used for all Bond and BondAngle objects.
Parameters (excluding bond strengths) are from:
Comparison of simple potential functions for simulating liquid water
Jorgensen WL, Chandrasekhar J, Madura JD, Impey RW, Klein ML
The Journal of Chemical Physics. 79 (2): 926–935 (1983)
The strengths provided are from:
https://lammps.sandia.gov/doc/Howto_tip3p.html
having converted from their units of kcal to our kJ.
Note that different values for bond strengths are given in the OPLSAA data
file, namely 2510.4 and 313.8 respectively."""
from MDMC.MD.force_fields.ff import WaterModel
from MDMC.MD.interaction_functions import (Coulomb, HarmonicPotential,
LennardJones)
from MDMC.MD.interactions import Bond, BondAngle, Dispersion, Coulombic
[docs]
class TIP3P(WaterModel):
"""
TIP3P force field - LJ, Coulombic, fixed bond lengths and angles
"""
n_body = 3
@property
def interaction_dictionary(self):
# Charge Parameters
q_O = -0.834 # e
q_H = abs(q_O/2) # e
# LJ Parameters
sigma = 3.151 # Ang
epsilon = 0.6363 # kJ mol^-1
# Bond Parameters
r_OH = 0.9572 # Ang
f_OH = 1882.8 # kJ mol^-1 Ang^-2
# Bond Angle Parameters
a_HOH = 104.52 # deg
f_HOH = 230.12 # kJ mol^-1 rad^-2
return {
(Coulombic, ('O',)): Coulomb(q_O),
(Coulombic, ('H',)): Coulomb(q_H),
(Dispersion, ('O', 'O')): LennardJones(epsilon, sigma),
(Bond,
('H', 'O')): HarmonicPotential(r_OH, f_OH, interaction_type='bond'),
(BondAngle,
('H', 'O', 'H')): HarmonicPotential(a_HOH, f_HOH,
interaction_type='angle')}