The dissociation energies from all rovibrational levels of H2 and D2 in the ground electronic state are calculated with high accuracy by including relativistic and quantum electrodynamics (QED) effects in the nonadiabatic treatment of the nuclear motion. For D2, the obtained energies have theoretical uncertainties of 0.001 cm(-1). For H2, similar uncertainties are for the lowest levels, while for the higher ones the uncertainty increases to 0.005 cm(-1). Very good agreement with recent high-resolution measurements of the rotational v = 0 levels of H2, including states with large angular momentum J, is achieved. This agreement would not have been possible without accurate evaluation of the relativistic and QED contributions and may be viewed as the first observation of the QED effects, mainly the electron self-energy, in a molecular spectrum. For several electric quadrupole transitions, we still observe certain disagreement with experimental results, which remains to be explained.

The dissociation energy of molecular hydrogen is determined theoretically with a careful estimation of error bars by including nonadiabatic, relativistic, and quantum electrodynamics (QED) corrections. The relativistic and QED corrections were obtained at the adiabatic level of theory by including all contributions of the order α 2 and α 3 as well as the major (one-loop) α 4 term, where α is the fine structure constant. The computed α 0 , α 2 , α 3 , and α 4 components of the dissociation energy of the H 2 isotopomer are 36118.7978 (2) 2

A method is developed for automatic generation of intermolecular two-body, rigid-monomer potential energy surfaces based on symmetry-adapted perturbation theory (SAPT). It is also possible to substitute SAPT interaction energies by values computed using sufficiently high-level supermolecular methods. The long-range component of the potential is obtained from a rigorous asymptotic expansion with ab initio computed coefficients which seamlessly connects to SAPT interaction energies at large separations. An accompanying software package has been developed and tested successfully on eight systems ranging in size from the Cl-HO dimer to the cyclotrimethylene trinitramine dimer containing 42 atoms total. The potentials have a typical fit error of about 0.2 kcal/mol in the negative energy region. The accuracy may be further improved by including off-atomic sites or increasing their number. All aspects of potential development were designed to work reliably on a broad range of systems with no human intervention.

The refractive index n of gaseous helium can be measured by optical interferometry so accurately that it can be used to establish a pressure standard which is expected to be superior to the current standard based on the height of a mercury column. The new standard requires knowledge of the dynamic polarizability of helium atom with accuracy significantly higher than obtainable in the best experiments, but possible to achieve computationally. Calculations of this quantity are presented at relativistic and quantum electrodynamics levels of theory including relativistic nuclear recoil effects. The uncertainties of the results are carefully estimated. Our recommended value of the dynamic polarizability at the He-Ne laser wavelength of 6329.908Å, equal 1.391 811 97(14) a.u., has uncertainty about two orders of magnitude smaller than that of the most precise measurements and is sufficiently accurate to establish a new pressure standard. Purely ab initio values of the refraction coefficient n are computed using our polarizability and literature values of magnetic susceptibility and dielectric virial coefficients. It is shown that n − 1 can be predicted by theory as a function of pressure and temperature with uncertainty of 1 ppm for pressures up to 3 MPa.