Incorrect dipole moment of TIP4P/2005 water molecule

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1

Dear all,

I am trying to calculate dipole moment of TIP4P/2005 water molecule in a bulk systems using LAMMPS (16 Mar 2018). It is reported in the literature that the dipole moment of TIP4P/2005 water should be around 2.34 Debye at room temperature, whereas I am getting it around 3.2 Debye. I get the same dipole moment for an individual water molecule whether I run the simulation for single molecule or for a bulk system. I would like to know where am I making mistake in the input script? Could you please help me to sort this out? The following is my input script that I have used to calculate the dipole moment of single water molecule:

dimension 3
units real
boundary p p p
kspace_style pppm/tip4p 1e-4

atom_style full
pair_style lj/cut/tip4p/long 1 2 1 1 0.1546 8.5 8.5
bond_style harmonic
angle_style harmonic
pair_modify tail yes

read_data one.data

pair_coeff 1 1 0.1852055 3.1589 #O-O
pair_coeff 1 2 0.00000 0.000 #O-H
pair_coeff 2 2 0.00000 0.00000 #H-H

bond_coeff 1 0.00 0.9572
angle_coeff 1 0.00 104.52

# create groups ###
group ox type 1
group hy type 2
group water type 1 2

set group ox charge -1.1128
set group hy charge 0.5564

velocity all create 298.0 4928459 rot yes dist gaussian

neighbor 3.0 bin
neigh_modify delay 1 every 1 check yes

timestep 1.0

fix constrain water shake 1.0e-3 2 0 b 1 a 1
fix corre water nvt temp 298.0 298.0 1.0

#dipole calculation
compute dip water chunk/atom molecule
compute myChunk water dipole/chunk dip
fix diplo water ave/time 1 1 1 c_myChunk[*] file dipole-bulk.out mode vector

dump trje all custom 1 watnvt.lammpstrj id type x y z vx vy vz
dump_modify trje sort id
run 10

2

Dear all,

I am trying to calculate dipole moment of TIP4P/2005 water molecule in a bulk systems using LAMMPS (16 Mar 2018). It is reported in the literature that the dipole moment of TIP4P/2005 water should be around 2.34 Debye at room temperature, whereas I am getting it around 3.2 Debye. I get the same dipole moment for an individual water molecule whether I run the simulation for single molecule or for a bulk system. I would like to know where am I making mistake in the input script? Could you please help me to sort this out? The following is my input script that I have used to calculate the dipole moment of single water molecule:

the problem here is, that the position of the negative charge in TIP4P water is not on the location of the oxygen atom but on a point called M. as a special optimization this point M is not explicitly used when using a “tip4p” pair style, but computed on the fly for computing the coulomb interactions only. thus compute dipole/chunk cannot give the correct result (it would need to do the same transformation).

however, there is little point to use compute dipole/chunk, since you are using a rigid water potential and thus the dipole moment does not change. you can just compute it directly from the geometry of a single water with a pen and a piece of paper.

axel.

3

Dear Axel,

Thank you for your response. Now I got it! I need to do some post processing to transform the coordinates to get the location of point M, and then do the calculation for dipole moment using separate code. Actually, I do not need to calculate dipole moment of a single water molecule (as you said that can be done manually). I was just checking the input script as I was not getting the correct result for my actual system. Thank you again!

Regards,
Manish