m = 1.
k = 1.

# analytic solution for 1D monoatomic chain
omega(x) = 2.*sqrt(k/m) * abs(sin(pi*x))

set xlabel 'q'
set ylabel '{/Symbol w}'
set xr [-0.5:0.5]
set yr [0:2.5]

set format y '%.1f'
set term post enha color 20
set out 'pdisp.eps'
set key top left
set xtics ("-{/Symbol p}/a" -0.5, "{/Symbol G}" 0.0, "{/Symbol p}/a" 0.5)
set samples 101
set grid xtics lt 9

plot omega(x) w l lt 1 lc 3 t 'Theory', 'disp.dat' u 1:5 w p pt 7 ps 1.0 lc 1 t 'MD'