Non-Born-Oppenheimer calculations of the pure vibrational spectrum of T2 including relativistic corrections

We report very accurate calculations of the complete pure vibrational spectrum of the T-2 molecule with an approach where the Born-Oppenheimer (BO) approximation is not assumed. As the considered states correspond to the zero total angular momentum, their non-BO wave functions are spherically symmetric and are expanded in terms of all-particle, one-center, spherically symmetric explicitly correlated Gaussian functions multiplied by even nonnegative powers of the internuclear distance. The nonrelativistic energies of the states obtained in the non-BO calculations are corrected for the relativistic effects of the order of alpha(2) (where alpha is the fine structure constant) calculated as expectation values of the operators representing these effects. (C) 2014 AIP Publishing LLC.