"""A module for defining the SPCE forcefield
This definition of the SPCE 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, SPCE
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:
The missing term in effective pair potentials
H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma
J. Phys. Chem. 1987, 91, 24, 6269–6271
The strengths provided are the same as those used for SPC, 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 SPCE(WaterModel):
"""
SPCE force field - LJ, Coulombic, fixed bond lengths and angles
"""
n_body = 3
@property
def interaction_dictionary(self):
# Charge Parameters
q_O = -0.8476 # 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')}