If you know the structure draw it using chem draw and do the file conversion, then you can open open it using GaussView , then open it using word pad you will get the coordinates. Or draw it using gaussView. 

Thanks chammi but my question is how should give the input of a dimer e.g. two molecule bond angle, dihedral  angle etc.

I supoose there's no xray structure of this dye? If so, look in xray database for something structurally similar and see if it forms dimers and how. If not, take a look at the crystallographic cell, that may give you a hint on how it's interacting with neighbouring molecules.

Hi Debabrata,

It would be of help if you tell us which dye are you dealing with. If the dye is plane or has a plane region holding part of the chromophore, you  can just place two optimized monomers, on in front each other at 2.6-3.2 Angstroms of distance, holding the two pi systems planes parallel; then you can start an ordinary geometry optimization. You can try some different orientation and evaluate which is the most stable final structure.

You can further improve the geometry of  encounter by considering this:

Make a graph of the electrostatic potential (I don't know what are you using for visualization, maybe gaussview, if not you can generate the electrostatic potential cube with cubegen utility and then use it with gabedit, VMD, molden or other visualization software, capable of opening gaussian cubes). Now take a look to the asymmetry in the charge distribution. You can orient the two monomers by ensuring that the most positive part of on monomer lies in front of the most negative of the other.

Another possibility is to follow Bartosz's suggestion, in case you were able to find X-ray evidence for very closely related molecules

Hi Debabrata, Hi everybody

If you know your structure, draw it by "Gauss view" , which can be used in Gaussian jobs, too

Hi Bartosz,

As you mentioned, i download cif file of the chromophore than i saw the crystal structure there i was pi-pi interaction and parallel orientation of two monomer. Based on that i generated input  having parallel conformation of homo monomers.  Now, step by step i am optimizing the ground state geometry and excited state geometry of dimer. Now, my confusion is that its crystal structure so how much this is relevant o dimers.

Thanks.......a lot

Thanks Mariano,

I am using coumarin as chromophore.I placed two monomers at parallel and anti parallel orientation and starting to optimizing the ground state. But the problem is that when optimized ground state structure is very different than input i.e.making an angle of 40 degree to each other. . So if i change suppose the distance between monomers and optimize the geometry so how much accurate is this, as ultimately i will get optimized structure of 40 degree.