With a plane wave code you're stuck to 3D periodicity, even for 0D systems (molecules).

You are right that the energy levels should be discrete. You should use a gamma-point only calculation. You should also make sure your unit cell is large enough so that the molecules in neighboring cells don't interact. (run some tests on making your cell bigger - a supercell approach could be one way). 

Thank you Gaumans for quick response.

I have used gamma point only calculation for my structure. Should I increase the number of repeating units or just increase the cell length of the unit cell? With increase in number of units number of molecules will also increase hence the computation time.I wish to get fruitful results in less computation time. 

Thank you Sir Behnam Farid for explaining the concept so elaborately.

Thank you Gaumans for the suggestion of using gamma points only calculation in case of isolated molecules.

Now, I have got a better insight into the whole concept and approach. I attach here two references containing calculations based on CASTEP. Kindly, go through the computational methodology part in these papers.

 i am the new user of castep software.could you tell me how to create  nayf4:yb3+.kindly help me.

Dear Amruta, 

I am also new to this software but from whatever I have learnt I can say that  you need to know the fractional coordinates of all the atoms in the crystals system first then follow:

Build->Add atoms;  there you can give the coordinates by selecting specific atoms then the crystal structure will appear in the 3D atomistic interface.

thanks sapna madam, but mam how to doping .what i ve read there is no direct doping by this software.

so we ve to create a super cell.how to be create.

There is some option to replace an atom by some other atom,please check the tab modify. For creating a supercell you go to Build tab you will find the option there itself. Go through the tutorials in order to get the correct guidelines.

thank you sapna ma'm.i ll do it.

The calculation procedure of different properties in supercell approach is same as that of normal unit cell. However, since a supercell contains a large number of atoms in a cell, generally a comparatively less K-points and cutoff energy provides more accurate results than the normal cell. You can follow the below article for convenience ....

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