# AMBER Archive (2004)Subject: RE: AMBER: Dipole moment changes during MD

From: Ross Walker (ross_at_rosswalker.co.uk)
Date: Fri Apr 09 2004 - 16:56:48 CDT

Dear Jiten,

The way I implemented the measurement of dipoles in Amber 8 was to calculate
them on the fly by calling a routine every time the results MD results were
written to the mdout file. The code would look at the group specification
for &dipoles in the input file and use this to determine how many different
sets of dipoles to calculate and what atoms to include in each. For each
dipole group chosen the code then looks up the X Y Z coordinates and charges
(and also induced dipoles if running a polarisable calculation) for each
atom in the group.

Once you have this data calculating the dipole moment of the group is fairly
simple. However, since the dipole moment is only non-ambiguous when the
group is neutral the centre of mass of the group is first calculated. The
dipole moment of the group is then calculated with respect to the group's
centre of mass in order to remove this ambiguity. Note, if your groups are
all neutral then you can skip this step.

So, centre of mass for each variable is:
N
X = (1/M)Sum{M(i)*X(i)}, Y = ..., Z = ...
i=1

Next step is to subtract the centre of mass from the coordinates of each
atom in your group. Then calculate the permanent dipole moment:
N
Px = Sum{q(i)*x(i)}, Py = ..., Pz = ...
i=1

This gives you the vector for your dipole moment. The magnitude of which
ptot = sqrt(Px^2+Py^2+Pz^2).

So, since your charges don't change in your simulation you should be able to
post process your mdcrd file to obtain the permanent dipole moment. Note: Be
careful with the units to ensure you get the answer in Debye.

Note: 1 Debye = 0.2082267 electron angstroms
Therefore 1 eA = 4.80246D

With regards to calculating the dipole moment of your water I'm not sure
what you hope to see by this. You should see the correct dipole magnitude
and relative direction for each individual water molecule but the overall
dipole moment for your solvent will not be the same as the dipole moment for
a single water molecule since the waters will fluctuate over time. Hence at
each point in time the individual dipoles will all be aligned in different
directions and thus will cancel out. Unless of course something interesting
is going on that is introducing order into your solvent and causing the
dipoles to align.

I hope this helps
All the best
Ross

/\
\/
|\oss Walker

| Department of Molecular Biology TPC15 |
| The Scripps Research Institute |
| Tel:- +1 858 784 8889 | EMail:- ross_at_rosswalker.co.uk |
| http://www.rosswalker.co.uk/ | PGP Key available on request |

_____

From: owner-amber_at_scripps.edu [mailto:owner-amber_at_scripps.edu] On Behalf Of
Jiten
Sent: 09 April 2004 11:05
To: amber_at_scripps.edu
Subject: Re: AMBER: Dipole moment changes during MD

Dear Dr. Ross Walker,

Apart from checking the density of the solvent during the MD, I also wish to
check the dipole as it also represents a property of the liquid. For example
WATBOX216 (given along with the amber7 for TIP3 water model) must give a
dipole moment of ~3 D. I am wondering how could I calculate the actual
dipole moment of a solvent system.

At the moment I use amber7 (planning to purchace amber8 soon).

Your suggestions would be highly appreciated.

With regards,

JIten

----- Original Message -----
From: Ross <mailto:ross_at_rosswalker.co.uk> Walker
To: amber_at_scripps.edu
Sent: Saturday, April 10, 2004 1:48 AM
Subject: RE: AMBER: Dipole moment changes during MD

Dear Jiten,

Which version of amber are you running? Amber v8 (Sander) includes the
facility for printing permanent and induced dipole moments for any groups
specified in the &dipoles namelist. This will print the dipole information
to the mdout file every NTPR steps. See section 5.6.17 of the amber manual.
Let me know if you have any problems getting it to work properly. I have
tested it with a range of molecules but never actually tried calculating the
dipole of the solvent. Note, if you are running a non-polarisable force
field then you will simply get the permanent dipole. If you are running a
polarisable force field then you will get permanent, induced and total
dipoles.

If you are using an older version of sander then you will have to do this
manually yourself. It is possible that you could use ptraj to post process
your trajectory file to calculate the permanent dipoles (but not the
inducible ones) although I have not tried this.

All the best
Ross

/\
\/
|\oss Walker

| Department of Molecular Biology TPC15 |
| The Scripps Research Institute |
| Tel:- +1 858 784 8889 | EMail:- ross_at_rosswalker.co.uk |
| http://www.rosswalker.co.uk/ | PGP Key available on request |

_____

From: owner-amber_at_scripps.edu [mailto:owner-amber_at_scripps.edu] On Behalf Of
Jiten
Sent: 08 April 2004 18:31
To: amber_at_scripps.edu
Subject: AMBER: Dipole moment changes during MD

Dear Amber community,

Could anyone suggest me how I can calculate the dipole moment of the solvent
box during the course of the MD run / snapshots of MD run.

N. Jiten Singh
C/O Prof. Kwang S. Kim
Department of Chemistry
Pohang University of Science and Technology