m = 1.
M = 2.
k = 1.

omp(x) = sqrt( k*(m+M)/(m*M)*(1. + (1.-4*m*M/(m+M)**2*(sin(pi*x))**2)**0.5))
omm(x) = sqrt( k*(m+M)/(m*M)*(1. - (1.-4*m*M/(m+M)**2*(sin(pi*x))**2)**0.5))

set xlabel 'q'
set ylabel '{/Symbol w}'
set xr [-0.5:0.5]
set yr [0:2.0]

set format y '%.1f'
set term post enha color 20
set out 'pdisp.eps'
set key top left
set xtics ("-{/Symbol p}/2a" -0.5, "{/Symbol G}" 0.0, "{/Symbol p}/2a" 0.5)
set samples 101

set grid xtics lt 9

plot omp(x)     w l lt 1 lc 3 t 'Theory',\
     omm(x)     w l lt 1 lc 3 t '',\
     'disp.dat' u 1:5 w p pt 7 ps 1.0 lt 1 t 'MD',\
     ''         u 1:6 w p pt 7 ps 1.0 lt 1 t ''