HYDROGEN MOLECULE IN MAGNETIC-FIELDS - THE GROUND-STATES OF THE SIGMA-MANIFOLD OF THE PARALLEL CONFIGURATION

The electronic structure of the hydrogen molecule is investigated for the parallel configuration. The ground states of the Sigma manifold ar e studied for ungerade and gerade parity as well as singlet and triple t states covering a broad regime of field strengths from B=0 up to B=1 00 a.u. A variety of interesting phenomena can be observed. For the (1 ) Sigma(g) state we found a monotonous decrease of the equilibrium dis tance and a simultaneous increase of the dissociation energy with grow ing magnetic-field strength. The (3) Sigma(g) state is shown to develo p an additional minimum which has no counterpart in field-free space. The (1) Sigma(u) state shows a monotonous increase in the dissociation energy with a first increasing and then decreasing internuclear dista nce of the minimum. For this state the dissociation channel is H-2-->H -+H+ for magnetic field strengths B greater than or similar to 20 a.u. due to the existence of strongly bound H- states in strong magnetic f ields. The repulsive (3) Sigma(u) state possesses a very shallow van d er Waals minimum for magnetic-field strengths smaller than 1.0 a.u. wi thin the numerical accuracy of our calculations. The (1) Sigma(g) and (3) Sigma(u) states cross as a function of B and the (3) Sigma(u) stat e, which is an unbound state, becomes the ground state of the hydrogen molecule in magnetic fields B greater than or similar to 0.2 a.u. Thi s is of particular interest for the existence of molecular hydrogen in the vicinity of white dwarfs. In superstrong fields the ground state is again a strongly bound state, the (3) Pi(u) state.