"""A module for defining the SPC forcefield
This definition of the SPC 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, SPC
itself is a rigid model, and in order to replicate this a constraint algorithm
should be used for all Bond and BondAngle objects.
Parameters are from:
A molecular dynamics simulation of a water model with intramolecular degrees of freedom
O. Teleman, B. Jonsson, S. Engstrom
Mol. Phys. 60(1), 193-203 (1987)
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 SPC(WaterModel):
"""
SPC force field - LJ, Coulombic, fixed bond lengths and angles
"""
n_body = 3
@property
def interaction_dictionary(self):
# Charge Parameters
q_O = -0.82 # e
q_H = abs(q_O/2) # e
# LJ Parameters
sigma = 3.166 # Ang
epsilon = 0.6502 # kJ mol^-1
# Bond Parameters
r_OH = 1.000 # Ang
f_OH = 4637. # kJ mol^-1 Ang^-2
# Bond Angle Parameters
a_HOH = 109.47 # deg
f_HOH = 383. # 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')}