CCL: difference between CI single and double excitations



Dr. Close & Dr. Sukumar,

My special thanks to you for responding. I actually performed ground state optimization on a system using a DFT technique. After that  i want to optimize the excited states of that system, for that i wanted to use CI method. I was confused with the CIS and CID methods and which one to use and which one is more accurate. And one more question i would like to ask you is about the restricted CI and unrestricted CI, the difference between the two. Kindly help me in this regard. I thank you one again for your help

Regards,
Aditya Khandavelli.