I'm actually doing the relaxation of the molecule with pw.x ("relax") and then I run a vibrational analysis of the phonons with ph.x. Finally, using dynmat.x I obtain the IR activities for each one of the modes.

For ethane the results are:

# mode    [cm-1]    [THz]         IR

  1           -29.05   -0.8708   0.0000

  2             -3.80   -0.1140   0.0000

  3             1.33     0.0398   0.0000

  4             3.75     0.1124   0.0000

  5           15.44     0.4630   0.0000

  6           38.59     1.1570   0.0000

  7         295.66     8.8636   0.0000

  8         797.29   23.9021   0.1121

  9         797.60   23.9114   0.1118

  10       990.01   29.6799   0.0000

  11     1175.17   35.2306   0.0000

  12     1176.33   35.2655   0.0000

  13     1355.15   40.6265   0.0347

  14     1366.58   40.9689   0.0000

  15     1449.44   43.4532   0.0169

  16     1449.83   43.4648   0.0011

  17     1449.99   43.4695   0.2195

  18     1450.64   43.4892   0.2392

  19     2949.57   88.4260   1.4643

  20     2963.43   88.8415   0.0000

  21     3011.98   90.2970   0.0001

  22     3013.35   90.3378   0.0002

  23     3036.37   91.0280   1.3616

  24     3037.35   91.0573   1.3605

(I'm doing a summation of the frequencies and dividing by 2, the final result is 0,3137 eV, but I expected ~1.98)

How can I use the IR data to calculate the zero point energy of this molecule?

Thanks!