Hello Isa,

Can be considered the charge of the system being 0 and let the two entities the pyridine with H and acetoacetate as they are and compute an optimization? Gaussian should calculate and somehow know about how charge is distributed between the two. Just an opinion :)

It should be done in the way as Marcel Mirel Popa said. However, there is a provision to assign charge and multiplicity to different fragments in your system. In that case, you need to define different layers constituting those fragments: high, medium, low etc. It is called the multilayered ONIOM calculations (http://gaussian.com/oniom/).

Mr. Man,

Via ONIOM.

You shouldn't need to assign anything apart for the total charge of the system. If the system you want to simulate really contains two molecules with different charges, this charge distribution should manifest itself during the Gaussian SCF energy minimisation procedure. If it doesn't, you need to question the methodology you are using, rather than "forcing" the system to reflect the charge separation you desire.

Hi Isabela,

I agree with Andrew. All you can do is place the hydrogen on the pyridine and have the oxygen on the alpha acetoacetate without its proton. You may consider optimizing pyridinium ion (charge = 1 and multiplicity = 1) and the alpha acetoacetate ion (charge = -1 and multiplicity = 1) first and then putting them together like you think they should be and then optimizing the system which will have charge = 0 multiplicity = 1. Finding absolute energies of a single system does not have meaning anyway. I would think that you would want to look at the energy of the ion pair relative to the individual ions. Of course, this calculation is in the gas phase and carrying out the calculation in the liquid phase is obviously far more complicated.