You can easily get the information about functional and basis set in journals

HF functional is not suitable for calculating many kinds of system. Because it cannot represent correlation of electrons.

The most popular and general functional in current computational chemistry is hybrid functional, which is composed of HF exchange and correlation function.

It should give better results than HF functional.

However, you should remember one thing. If your system shows charge transfer phenomona, you need to use long range corrected hybrid functional such as CAM-B3LYP, wB97xD.

If your system shows general p-p transition behavior, you can use any hybrid functional

Hope this help 

Dear sunwoo kang,

Thanks for your reply it helps me a lot....

Dear Venkatesan Ragavendran,

You optimized your molecules using HF method. Actually, HF method will yield results with less accuracy. Because it doesn't include polarization and diffuse functions. on the otherhand DFT method is universally accepted to treat the electronic structure of the molecule ranged from medium to large sized molecules. So try to make a calculation once again with DFT/6-311++G(d,p). I hope this will give you an exact result. Also make sure that your compound is high purity. suppose if impurities present, it will affect the band gap of the molecule too.

Thanking you,

by

Karthick

Dear karthick, 

I am following the dft methods only for long time and have optimized over hundreds of molecules. This is the first time I am encountering these kind of problems. Even this time I have checked with DFT methods still not working. Now today I have got some idea i will try them and will inform you whether i got succeeded or not.

Thanks for your interest....

Dear sunwoo kang,

thanks for your help. Your idea worked really and I got solved my problems.

Many a thanks...........

Hi renjith,

I have used DFT only and got good results. Do you have any comment regarding my issue. Feel free to comment.

hi renjith,

What is the difference between TD-DFT AND CAM-B3LYP can you differentiate?

quite depends on your compounds and there atoms . but one of the best method is CAM-B3LYP for TD-DFT study and you can use the LANL basis-set . such as LANL2DZ or LANL2MB ! but you should  attention to atoms basis-set in data bases and select one of them.