Hi Haldar,

The charges you mention are formal charges, and do not necessarily reflect the actual electron density of the molecule. If it is only a charge-separted initial guess you want to generatate, and then let the system converge to whatever electron density is optimal, you can use the guess=fragment option in gaussian. If you want to deal with a charge separated system all through the calculation (say between a negative acceptor and a positive donor) you would need to move into something like constrained dft in qchem.

Dear halder,

one can include charge of the entire molecule in the input file and normally a gaussian input file wont contain individual atoms' charges. so from my experience it cant be given. But there may be some experts in the same field and we hope that sumone may give suitable answer.

best wishes...

Thanks Ragavendran. For the same reason I have posted the question.

Hi Haldar,

The charges you mention are formal charges, and do not necessarily reflect the actual electron density of the molecule. If it is only a charge-separted initial guess you want to generatate, and then let the system converge to whatever electron density is optimal, you can use the guess=fragment option in gaussian. If you want to deal with a charge separated system all through the calculation (say between a negative acceptor and a positive donor) you would need to move into something like constrained dft in qchem.

Hi rafael, 

u r right and hope it helps halder for charge seperated system calculations.

Thank you Rafael. I got your point. Currently I am using the fragmentation methods. Lets see any differences comes or not? 

As noted before, there is no way to specify the charge in an individual atom. Simply draw the molecule correctly in  GaussView (for example). The orbitals will be constructed around those nuclei, and the wavefunction and electron density will be the optimal ones for that geometry. Any assigned charges - either formal charges that we like to draw on specific atoms or charges calculated from a wavefunction (like Mulliken or NBO charges) are the RESULTS of a specific electron distribution, NOT a pre-determined input into a calculation.

How is it possible to study ionic systems? In such systems, we have to fix charge on the cation and anion systems right?

Hi agoChanchal Haldar , 

first, you can use ONIOM method to split basis sets of your compound. after that; the atom that you want to assign its charge keep it alone in layer from three layers. however , input specific charge of atom in the charge option from the gaussian panal, all this before runing calculation.

Rangavendran sir is totally correct. U need to compile up the overall charge for a transition state or complex or fragment to put in Gaussian calculation. Individual charge assignment is not calculated alone as it is not a specified electron cloud for mathematical dimension approach.