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