The original model of Das et al. is modified in extending the electron
-ion interaction on a three-body forces and including the crystal equi
librium condition to reduce one independent parameter. We studied the
phonon dispersion relations along the three principal symmetry directi
ons i.e. [xi, 0, 0], [xi, xi, 0] and [xi, xi, xi] and theta-T curves o
f alkali metals, Na, K, Rb, Cs and Li. There is close agreement betwee
n the computed results and the experimental observations.