Dear Zakir Ali

LanL2DZ may work, but for accurate energy it is small for iodine.

We used WTBS for tellurium (iodine neighbour), here the details:

Article Structural and Electronic Properties of Boranes Containing B...

Best, Pablo

Dear Zakir Ali,

my group works both experimentally and theoretically on the chemistry of iodine (and in general dihalogens/interhalogens) since several decades. As far as my experience is regarded, LANL2DZ and the more recent LANL08 basis sets perform quite well in modeling the structural and vibrational (Raman) features of compounds involving iodine.

The literature on this topic is vast: just to cite a recent work, please refer to Article Role of the Solvent in the Reactivity of Bis-4-imidazoline-2...

I hope this may help and provide at least a starting point.

I would use def2-TZVP if you are looking for a basis set of triple-zeta quality that also accounts for relativistic effects.