CCL:G: fixing charges on atom in gaussian03 package

[Ivo Jacobs <jacobs.ivo=gmail.com>]

 Sent to CCL by: Ivo Jacobs [jacobs.ivo(~)gmail.com]
 Partha Biswas partha361~~gmail.com wrote:
>  am an user of Gaussian03 package. I want to specify
>  charges on atoms. The
>  overall charge of the molecule is 0 but for molecules such as amino
>  acid which
>  I want to be in zwitterionic form that is the amine (-NH2) group will
>  be NH3+
>  and the carboxylic acid (-COOH) group will stay as COO-. Thus I have to
>  specify
>  the charge on nitrogen atom (N) as +1 and oxygen atom (O) of COO- group
>  as -1.
>  After that I will run the full optimization (with keyword fopt) in gas
>  phase.
>  Though gas phase amino acid zwitterion is not stable and it will
>  actually
>  transform to neutral form, but if I can fix charges then probably not
>  transform
>  to its neutral from.
>  
>  I will be helpful if you kindly help me out of this problem.
>  
You don't have to specify the charges in this case. As
 long as you put
 the proton where you want it, so on the nitrogen, Gaussian will
 distribute the electrons properly, resulting in a positive charge on
 the
 nitrogen, and a negative charge on the oxygen.
Unless the proton is very close to the oxygen it will not go back to
 the
 oxygen automatically.
 HTH
 Ivo