Explicitly correlated Gaussian functions in variational calculations. Microhartree accuracy of the helium dimer energy

The results of variational calculations on the helium dimer at the van der Waals minimum are presented. A wavefunction in the form, of a linear combination of explicitly correlated Gaussian functions is employed. It enables one to achieve an accuracy of a fraction of a microhartree. The lowest upper bound to the ground state energ computed at an internuclear distance of R = 5.6 bohr is -5.807483400 hartree. The estimated error in the energy is about 0.05 cm(-1). The predicted lower bound to the potential well depth is 10.94 K.