The electronic structure of the hydrogen molecule is investigated for the parallel configuration. The ground states of the ⌺ manifold are studied for ungerade and gerade parity as well as singlet and triplet states covering a broad regime of field strengths from Bϭ0 up to Bϭ100 a.u. A variety of interesting phenomena can be observed. For the 1 ⌺ g state we found a monotonous decrease of the equilibrium distance and a simultaneous increase of the dissociation energy with growing magnetic-field strength. The 3 ⌺ g state is shown to develop an additional minimum which has no counterpart in field-free space. The 1 ⌺ u state shows a monotonous increase in the dissociation energy with a first increasing and then decreasing internuclear distance of the minimum. For this state the dissociation channel is H 2 →H Ϫ ϩH ϩ for magnetic field strengths Bտ20 a.u. due to the existence of strongly bound H Ϫ states in strong magnetic fields. The repulsive 3 ⌺ u state possesses a very shallow van der Waals minimum for magnetic-field strengths smaller than 1.0 a.u. within the numerical accuracy of our calculations. The 1 ⌺ g and 3 ⌺ u states cross as a function of B and the 3 ⌺ u state, which is an unbound state, becomes the ground state of the hydrogen molecule in magnetic fields Bտ0.2 a.u. This 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 ⌸ u state.

Hydrogen molecule in a magnetic field: The lowest states of the Π manifold and the global ground state of the parallel configuration The electronic structure of the hydrogen molecule in a magnetic field is investigated for parallel internuclear and magnetic field axes. The lowest states of the Π manifold are studied for spin singlet and triplet(Ms = −1) as well as gerade and ungerade parity for a broad range of field strengths 0 ≤ B ≤ 100 a.u. For both states with gerade parity we observe a monotonous decrease in the dissociation energy with increasing field strength up to B = 0.1 a.u. and metastable states with respect to the dissociation into two H atoms occur for a certain range of field strengths. For both states with ungerade parity we observe a strong increase in the dissociation energy with increasing field strength above some critical field strength Bc. As a major result we determine the transition field strengths for the crossings among the lowest

Interaction between LiH molecule and Li atom from state-of-the-art electronic structure calculations Simultaneous optimization of exponents, centers of Gaussian-type basis functions, and geometry with fullconfiguration interaction wave function: Application to the ground and excited states of hydrogen molecule Helium dimer potential from symmetry-adapted perturbation theory calculations using large Gaussian geminal and orbital basis sets

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The rotation-vibration Hamiltonian for a neutral diatomic molecule in an external magnetic field is established by appropriately transforming the pseudoseparated Hamiltonian to the molecular fixed frame. The partially screened Born-Oppenheimer approximation is subsequently performed. By explicitly investigating the dissociation limit of the molecule into neutral atoms we are able to identify the motional screening operator which is responsible for the screening of the nuclei by the electrons against the magnetic field. A connection between the screening operator and the magnetic flux enclosed by the motion of the nuclei is established.