Dear Renjith,

You should calculate all possible conformations which can be achieved by conducting a rotation of important dihedral angles found in the compound.

The conformation with the lowest energy will be the global minimum structure.

I suggest that you run first optimize your compound using AM1 or PM3 then by DFT, followed by reaction coordinates of the angles by DFT.

I am attaching one of my papers as an example.

Hoping this will be helpful,

Rafik

Dear ranjith,

I hope Mr.Rafik's answer will be sufficient for your question. As he pointed out, try to do the rotation with any of the possible dihedral angles of your molecule. This is known as Potential energy surface scan(PES). You can see it in many of the computational chemistry books and gaussian manual. If you find nothing jus type it in the gaussian's official website you will get examples and procedures to how to do a scan. Still if you need anything please feel free to ask me.

All d best.

There is no straight cut way to do that. As others have already mentioned, you've to generate different guess structures by altering internal coordinates and have to optimize them separately. If number of isomers are low then it is possible to get them all by these way. Otherwise you'll need some efficient GA codes or codes to find out global minima. By the way Avogadro has a very basic conformational search function under extensions>Molecular mechanics tab, which can produce some reasonable guesses for further optimization in higher level if your molecule is not complex enough.

Dear Turbasu Sengupta, 

Can you explain further about that GA codes and also about that molecular mechanics tab? Which software you are using and how to search for that extensions tab?

Kindly clarify

Dear Venkatesan,

I don't use any GA codes for my own research but I know some research group extensively use those which are mostly in house and not for others. However, I'm aware of one open source software named 'ASE(Atomic simulation Enviornment)' which can be easily installed in Linux. This software provides GA as well as Global optimization codes. Link is given below:

https://wiki.fysik.dtu.dk/ase/tutorials/tutorials.html

Also, About conformer search in Avogadro please see the following link:

http://manual.avogadro.cc/content/7-optimizing-geometry/2-conformers.html