Hi!

I wonder, why you try to calculate LSPR frequency using the effective mass. Usually one uses Mie theory to calculate the analytic LSPR response where you get the absorption, scattering and extinction cross-sections for single nanoparticles. Then one uses the wavelength dependent refractive index and extinction coefficient (or the complex dielectric function) of the metal, which can be found at http://refractiveindex.info/?shelf=main&book=W&page=Rakic . If you have the possibility to use Mathematica program for example, then i would suggest you use a complimentary Mathematica notebook file which is available for download at http://www.cambridge.org/us/download_file/167789/ This file includes a nice routine and can calculate the exact Mie response and includes data for various metals. i think  a different chapter`s notebook file contained also the data for tungsten. The book itself:

Modern Introduction to Surface Plasmons

Theory, Mathematica Modeling, and Applications

Dror Sarid, University of Arizona

William Challener, Seagate Technology

is available here (http://www.cambridge.org/us/academic/subjects/physics/optics-optoelectronics-and-photonics/modern-introduction-surface-plasmons-theory-mathematica-modeling-and-applications) and is a very nice read. 

Anyhow I would use Mie theory and calculate the optical response using the dielectric function. I hope I could help you a little. 

This is helpful for me thanks..