Hi, Lammps users,
I simulate SPC/E water behavior on a rigid wall, but experienced weired results with respect to “special_bonds” command. (sorry for long explanation below)
I think: since H-O-H are bonded, it is unnecessary to compute the lj/coulombic forces between bonded atoms in each atoms. I delete the Lennard-Jones & Coulombic force between H and O bonded in each water molecules, and also the H-H interactions angled in the same water molecules. I only consider the interactions between atoms that belong to different molecules.
However i got some weird results: the water did not move at all after they achieve equilibrium at T=300k; the temperature is almost 300K; no water molecule escape.
I also tried simulation without special_bonds command, and it seems the water molecules are moving randomly as imagined at equilibrium at T=300K, which is more close to common sense.
I checked the mannual for details: The rationale for using these weighting factors is that the interaction between a pair of bonded atoms is all (or mostly) specified by the bond, angle, dihedral potentials, and thus the non-bonded Lennard-Jones or Coulombic interaction between the pair of atoms should be excluded (or reduced by a weighting factor). The 1st of the 3 coefficients (LJ or Coulombic) is the weighting factor on 1-2 atom pairs, which are pairs of atoms directly bonded to each other. The 2nd coefficient is the weighting factor on 1-3 atom pairs which are those separated by 2 bonds (e.g. the two H atoms in a water molecule). The 3rd coefficient is the weighting factor on 1-4 atom pairs which are those separated by 3 bonds (e.g. the 1st and 4th atoms in a dihedral interaction). Thus if the 1-2 coefficient is set to 0.0, then the pairwise interaction is effectively turned off for all pairs of atoms bonded to each other. If it is set to 1.0, then that interaction will be at full strength.
My thinking follows the above command explanation. But it seems this command did not show what i want.
My question is how to set the weighting factor for H-O-H which did not influence the results? Thanks!
The details of input are as following:
group hy type 1 #H of H2O
group ox type 2 #O of H2O
group Mg type 3
group O type 4
group water type 1 2
group boundary type 3 4
pair_style lj/cut/coul/long 10 10
kspace_style pppm 0.0001
pair_coeff 1 1 0 0 #hy-hy
pair_coeff 2 2 0.1554 3.166 #ox-ox
pair_coeff 3 3 9.03E-07 5.909 #Mg-Mg
pair_coeff 4 4 0.1554 3.166 #O2-O2
pair_modify mix arithmetic #shift yes
special_bonds lj/coul 0.0 0.0 0.5
velocity boundary set 0 0 0 units box
velocity water create 300 49875678 mom yes rot yes dist gaussian units box
fix 1 boundary setforce 0 0 0
fix 2 boundary rigid single
fix 3 water shake 1.00E-04 20 0 b 1 a 1 #H-O and H-O-H
fix NVT water nvt temp 300 300 100