Dear Monidipa Pramanik , your three questions are different and they have different answers too.
To do a Raman spectroscopy simulation, you should make a model of your molecule, how its atoms are bonded, what is the length and angle of each bond and then analyze the Raman active modes (how these bonds vibrate, bend, stretch, etc). Quantum mechanical methods, mostly Density Functional Theory (DFT) are used to simulate electronic and vibrational spectroscopies.
The second and third questions are related, but let us answer the third question before the second, by clarity:
When an electric field interacts with a metal nanoparticle, specially noble metals, their free electrons feel the electric field and they follow it, this generates a charge separation in the Ag nanoparticle, that chargue separation creates a new electric field that adds to the external one. The electric field in the close proximty to the Ag nanoparticle is enhanced and therefore any molecule close or adsorbed over the Ag nanoparticle would feel a strong field and therefore the Raman signal would be enhanced. To do this kind of simulations you need algorithms that allow you to solve near-field problems, basically the Mie scattering equation and the interaction of light (electric field) with materials.
The election of the electric field in these software packages should be the one that allow your experimental set-up, you can select the polarization and the kind of beam and of course the wavelength of the excitation wave. You also could choose the orientation or angle of incidence.
A good software to simulate this light mater interaction is Lumerical, but you can also find some other very good ones, such Opiwave, Meep, etc. Also Matlab and Comsol have packages for simulating light mater interaction.
Dear Monidipa Pramanik , to the best of my knowledge, what you are looking for is so-called Finite Difference Time Domain simulation which is abbreviated as FDTD simulation. This technique is widely used for inspecting the local electric field in proximity of plasmonic nanostructures like silver and gold nanoparticles. As Manuel elaborated, there are some softwares for running this simulation including Lumerical and Optiwave. Attached you can find a few pictures together with some articles and a researchgate discussion concerning your question which might be helpful.
Chapter FDTD Studies of Metallic Nanoparticle Systems
Article FDTD simulation of the optical properties for gold nanoparticles
Dear all, Lumerical and Optiwave and the packages for Matlab and Comsol are commercial solutions, most of them let you download a fully funtional trial version for a period of 30 days. All of them have a extensive knowledge base and tutorials, furthermore, there are plenty good youtube videos where users explain diverse cases.