Hydrogen molecule in a strong parallel magnetic field

We investigate the hydrogen molecule in a strong parallel magnetic field using a fully numerical Hartree-Fock approach. We find that for magnetic fields below 4.2×10⁴ T the ground state of H₂ is the strongly bound singlet state ¹Σg, for magnetic fields stronger than 3×10⁶ T the ground state of the molecule is the strongly bound triplet ³Πᵤ, and for magnetic fields between 4.2×10⁴ T and 3×10⁶ T the symmetry of the ground state is the triplet state ³Σᵤ, which is characterized by repulsion at intermediate internuclear distances and by a weak quadrupole-quadrupole interaction between atoms at large internuclear separation. In this region of magnetic field strength the hydrogen molecule is bound weakly, if at all; the hydrogen atoms behave like a weakly nonideal gas of Bose particles and can form a superfluid phase predicted in earlier works [Korolev and Liberman, Phys. Rev. Lett. 72, 270 (1994)]. For magnetic fields between ≈3×10⁵ T and 3×10⁶ T the triplet state ³Πᵤ is found to be metastable. This state may be responsible for an unknown excitonic line observed experimentally [Timofeev and Chernenko, JETP Lett. 61, 617 (1995)].