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 T₂ 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 α² (where α is the fine structure constant) calculated as expectation values of the operators representing these effects.