Dear all,
Can I perform simulation of system where the net charge is not zero? Any comments will be appreciated.
Kind regards
Bhaskar
Dear all,
Can I perform simulation of system where the net charge is not zero? Any
comments will be appreciated.
yes, why not?
how physically meaningful the results
will be depends on the individual system
and cannot be answered in general.
axel.
Dear Axel,
Thank you for your response. But then how the periodicity is handled in LAMMPS. My system consists of water and charged nano particle in it. How do I set this up in LAMMPS so that it takes care of the non zero net charge of the system. Your comments and suggestions are most welcome.
Kind regards
Bhaskar
Dear Axel,
Thank you for your response. But then how the periodicity is handled in
LAMMPS. My system consists of water and charged nano particle in it. How do
I set this up in LAMMPS so that it takes care of the non zero net charge of
the system. Your comments and suggestions are most welcome.
you have three choices:
1) add counter ions. that is what will happen in real life, too.
2) don't run a periodic system
3) ignore the fact that you have a non-zero total charge and
run a periodic system anyway. this will lead to a nonzero
component at k=0, which is going to be ignored and have
the effect of a neutralizing jellium (or an implicit counter
charge if you prefer to see it like that) in the neighboring cells.
it depends on what you want to do with your simulation
which of the three options is to be preferred.
axel.
And I want to clarify one thing. LAMMPS does
not "take care" of insuring you have zero net
charge. That's up to you.
Steve
Dear Steve/Axel,
Thanks for the clarifications. But I am confused now as Axel said that the non zero value at k=0 will be ignored and thereby neutralizing the system. This seems to me that LAMMPS takes care of non zero net charge on its own. Moreover, we can run a periodic simulation without paying attention that the net system charge is not zero.
To run my simulation(water + charged nano particle) I do not want to put counter ions manually myself. Which may create problems in whatever I want to look at. Your comments will be very much valuable.
kind regards
Bhaskar
Dear Steve/Axel,
Thanks for the clarifications. But I am confused now as Axel said that the non zero value at k=0 will be ignored and thereby neutralizing the system. This seems to me that LAMMPS takes care of non zero net charge on its own. Moreover, we can run a periodic simulation without paying attention that the net system charge is not zero.
There is no contradiction here. lammps will not neutralize your system. All that will happen is the ignoring of the divergent term at k=0 when computing kspace force contributions.
To run my simulation(water + charged nano particle) I do not want to put counter ions manually myself. Which may create problems in whatever I want to look at. Your comments will be very much valuable.
As I mentioned before, it is your choice to decide which approach is better. You are trading off convenience vs. realism. How much of an impact this has on the validity of your simulations is dependent on the problem and not the concern of the lammps code.
Axel
To run my simulation(water + charged nano particle) I do not want to put
counter ions manually myself. Which may create problems in whatever I want
to look at. Your comments will be very much valuable.As I mentioned before, it is your choice to decide which approach is
better. You are trading off convenience vs. realism. How much of an impact
this has on the validity of your simulations is dependent on the problem and
not the concern of the lammps code.
Axel
Indeed, any mechanism of surface charge generation in water must
necessarily produce counter-ions, as the system must remain
charge-neutral. A surface charge cannot exist in real life without
counter-ions (cf. any textbook on colloid science, e.g. the one by
Hunter).
Best regards,
Laurent