The S0(0) Roton Structure in Highly Compressed Hydrogen

By the application of quantum mechanical perturbation theory, a calculation of the S0(0) roton frequencies in hcp para-hydrogen is performed. Expressions for the second and third order term of the perturbative series have been derived, and their contribution included in the calculation. Calculations using literature anisotropic interaction potentials are presented, and the dependence of the frequencies on the different anisotropic potential terms is analyzed. The perturbative series is found to be convergent, and the third order terms to remain very small, up to very high density. In particular, it is shown that the calculated average frequency overestimate the experimental ones if this is calculated on the basis of gas phase potentials. The possibility of reproducing quantitatively the experimental data is discussed.