You probably have to do a unit conversion, since what units E and k are given in will depends on the code you are using. Often they are in eV and, respectively, in units of 2 pi/a (a being the real-space direction parallel to the k direction).

As a warning, note that effctive masses are quite sensitive to structure, sepecially to volumes (see attached).

Another thing that GW people may point out is that the mass has an important contribution from the (energy change of the imaginary part of the) self energy, if I remember correctly, which is (often poorly) approximated by V_xc in a DFT calculation.

E i.e energy is in eV and k (the x-parameter) is angstrom inverse (10^10).

To calculate effective mass, you will require h-bar, which has units in Joule*sec. Convert your E from eV to joule and recalculate your effective mass.

I hope this will give you the correct value.

Dear Madawa,

X-axis has a unit of per angstrom (k is a wave vector) and the unit along y-axis is eV (E representing energy in eV). In order to calculate the effective mass, first you need to convert eV to Joule and per angstrom to per meter so that you can pick directly the value of h_bar in the calculation which has a unit of Joule sec.

Thank you all for your kind supports. Finally I got reasonable results.Thanks again.