Computing Charges

Hi all

I am trying to simulate a Fullerene C720.
I have the set of Force fields parameters but I need the charges.
I was thinking about used fix_qeq but I have not clue how to extract
the charges because that fix does not dump information.

Do you have any clue about what can I do?

I will have also considered compute the mulliken charges in Gaussian
but the system is to large.
Do you have any suggestion or alternative ?

Thanks

John Michael

Hi all

I am trying to simulate a Fullerene C720.
I have the set of Force fields parameters but I need the charges.

if you have the force field parameters, you also have the charges. if
there are no charges in the force field mentioned, then they are zero.
setting them to something else would be a mistake. classical force
fields do not always need to have charges on (all) atoms.

I was thinking about used fix_qeq but I have not clue how to extract
the charges because that fix does not dump information.

no need. charges are accessible for atoms through dumps.
however fix qeq also needs parameters to work correctly.

Do you have any clue about what can I do?

I will have also considered compute the mulliken charges in Gaussian
but the system is to large.
Do you have any suggestion or alternative ?

should there actually be significant charges in the first place?
is the C720 fullerene a significant multipole?

axel.

Hi Axel

  1. We are using a generic force field that lack of charges (like Dreiding or UFF)
    therefore we need to compute these.

  2. What are those parameters that Fix qeq needs to work correctly? Can I use
    those from reax/c?

  3. Can I run reax/c and extract the charges for that simulation, considering
    that my system is not reactive ?

  4. About how important are the charges in C720 and large fulleneres, I am not sure.
    It is clear that for C60 there is not problem, due to the bond nature and the almost
    perfect symmetry. But the large fullerenes are not as symmetric (spherical symmetry)
    as C60, therefore the charge could be important. I do not know any experimental
    or theoretical paper that analyze this aspect.

thanks

John michael

Hi John Miciael,

1. We are using a generic force field that lack of charges (like Dreiding or
UFF)
therefore we need to compute these.

The papers that provide the force field parameters for Dreiding/UFF
should also have charge values. You should be required to compute
your own.

2. What are those parameters that Fix qeq needs to work correctly? Can I use
those from reax/c?

Please see the fix qeq/reax doc page. You should be able to, but you
should not have to.

3. Can I run reax/c and extract the charges for that simulation, considering
that my system is not reactive ?

That is not a good idea, since these force fields are designed to run
with their suggested charges.

Cheers,
Ray