New explicit Henon-Heiles potential

The potential is taken from the paper:

@article{davis_heller,
  author = {Michael J. Davis and Eric J. Heller},
  title = {Semiclassical Gaussian basis set method for molecular vibrational wave functions},
  publisher = {AIP},
  year = {1979},
  journal = {The Journal of Chemical Physics},
  volume = {71},
  number = {8},
  pages = {3383-3395},
}