Exponentially correlated Gaussian functions in variational calculations: Quadrupole moment for the ground state of helium dimer

Exponentially correlated Gaussian wave functions of 1200 terms were employe d to compute expectation values of the quadrupole moment, Theta, for the gr ound state of helium dimer. The quadrupole moment was calculated in the fra mes of the Born-Oppenheimer approximation for a wide range of internuclear distances, 0.0 less than or equal to R/a(0)less than or equal to 15.0. At t he equilibrium distance Theta=-6.2(1)x10(-4) ea(0)(2) and the estimated vib rational average amounts to -5x10(-7) ea(0)(2). For small R, the quadrupole shielding function, gamma(R)=-Theta(e)(R)/Theta(n)(R), was constructed, wh ich allowed the quadrupole shielding factor gamma(infinity) of beryllium at om to be estimated as equal to +1.5(1). As the wave functions employed are the most accurate variational wave functions for He-2, the quadrupole momen t results presented in this paper may serve as benchmarks. (C) 2000 America n Institute of Physics. [S0021-9606(00)30716-4].