Dear sir
You can go for staad which is widely used FEM analysis software.
Beside that it has the lowest cost in terms of license. There are
other software which are available in the market like- SAP, RISA , Ansis etc..
There is no Difference in calculation of seismic force for a geometric body. seismic force depends upon:
1)Mass of the Body.
2)Soil Type of that region.
3)Natural Frequency of the Body i.e. again depended on the mass and stiffness of the body.
4)Damping of soil i.e. depended on soil type...