Hi Malathi,

QSAR is very vast field to study and very important in drug designing approach. The attached link to the paper would help you to understand the concept

http://www.sciencedirect.com/science/article/pii/S1319610311000834

If you need a detailed study on these you can go through the following paper link: 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3211024/

Comfa deals with electrostatic and steric fields to correlate activity. Comsia utilizes SEAL similarity fileds that also inegrates hydrogen bonding potentials to correlate activity. The form of Lennard-Jones potential is varied among these two QSAR methods.

The acronym 3D-QSAR or 3-D QSAR refers to the application of force field calculations requiring three-dimensional structures of a given set of small molecules with known activities (training set). The training set needs to be superimposed (aligned) by either experimental data (e.g. based on ligand-protein crystallography) or molecule superimposition software. It uses computed potentials, e.g. the Lennard-Jones potential, rather than experimental constants and is concerned with the overall molecule rather than a single substituent. The first 3-D QSAR was named Comparative Molecular Field Analysis (CoMFA) by Cramer et al. It examined the steric fields (shape of the molecule) and the electrostatic fields which were correlated by means of partial least squares regression (PLS).

The created data space is then usually reduced by a following feature extraction (see also dimensionality reduction). The following learning method can be any of the already mentioned machine learning methods, e.g. support vector machines. An alternative approach uses multiple-instance learning by encoding molecules as sets of data instances, each of which represents a possible molecular conformation. A label or response is assigned to each set corresponding to the activity of the molecule, which is assumed to be determined by at least one instance in the set (i.e. some conformation of the molecule).

On June 18, 2011 the Comparative Molecular Field Analysis (CoMFA) patent has dropped any restriction on the use of GRID and partial least-squares (PLS) technologies.