[gmx-users] dipole moment of water using TIP3P model

[Justin Lemkul]

On 5/9/19 6:54 AM, Sabreen Farnaz wrote:
> Dear Users,
> It is known that electric dipole moment of a water monomer is
> experimentally 1.85 D and using the TIP3P model it's supposed to be 2.35D.
> I am using the TIP3P model for a water-based simulation in Gromacs 5.1.5.
> After applying sinusoidal electric fields in the Y axis, the norm of the
> dipole vector per water molecule using *gmx dipoles* is closer to 1.85 D
> and nowhere near 2.35D. My simulations are for 300K and 1 atm.
> Can anyone guess why the discrepancy? Is it because 2.35D is for a water
> monomer and my system has quite a large number of them(in the order of ten
> thousands)? But literature suggests that presence of water molecules should
> enhance the dipole moment. Or is 2.35D calculated under a different
> temperature/pressure conditions?

I don't know what your command was or how gmx dipoles might be 
calculating this quantity, but it is simply a fact that the dipole 
moment of rigid TIP3P is 2.35 D. It can't vary, by definition. You will 
not see any difference based on system size because the model is 
nonpolarizable - every TIP3P is identical, whether you have one or a 
million, so there is no dipole cooperativity.

-Justin

-- 
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Justin A. Lemkul, Ph.D.
Assistant Professor
Office: 301 Fralin Hall
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Virginia Tech Department of Biochemistry
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jalemkul at vt.edu | (540) 231-3129
http://www.thelemkullab.com

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