Please go through manual of any quantum chemistry software. From google also you may get sample input, ideas of method and basis set for Cl.

Its carbon tetra iodine C I4. And not

Ok. I overlooked it. For Iodine you can use stutgart-dresden basis set with a relativistic pseudopotential. Please look for ESML basis set Library.

Thank u Dr mandal. If u don't mind, can u post input file. I am new to Gaussian, hence not so expert. Moreover it does no't have an experimental geometry coordinates.

Thank u Dr Mandal ... I have done it...

Dr Mandal, you pl help in resolving this issue

CO2 belongs to D infinity H point group. i need to work in CO2 in D2H

how can i run using Gaussian 03 to obtain coordinates and other parameters in D2h point group sym

similarly N2O in C2V instead of C inf V

thanks

Sorry for Late reply. There is an option in Gaussian for Symmetry determination "Symm(PG=Group)". But I am not sure it works or not. Please test it. I hope you will not face any problem here. Because most probably, though CO2 and N2O possess Dinfh or Cinfv, Gaussian and other packages not uses the highest level of symmetry it always consider lower symmetry. In your case they are D2h and C2v. You can also test this. Hope this helps..