Short- and long-range binding of Be with Mg in the X1Σ+ ground state and in the A1Π excited state.

We present results of configuration-interaction (CI) computations of wavefunctions and of properties of the first two singlet states, X(1)Σ(+) and A(1)Π, of the, as yet unobserved, BeMg polar molecule, for internuclear distances in the range [2.5-1000] Å. The X(1)Σ(+) state is very weakly bound, (D(e) = 469.4 cm(-1) at R(e) = 3.241 Å), whereas the A(1)Π state, which correlates with the excited dissociation channel [Mg KL3s3p(1)P(o) + Be 1s(2)2s(2) (1)S], is bound rather strongly (D(e) = 19 394 cm(-1) (55.5 kcal/mol) at R(e) = 2.385 Å). The X(1)Σ(+) state supports 12 vibrational levels, for which vibrationally averaged dipole moments, <μ>(υ), were obtained, while 71 vibrational levels were found for A(1)Π. For the level (X(1)Σ(+)), <μ>(0) = 0.213 D. The υ(") = 7 and 8 X(1)Σ(+) vibrational levels are found to have the highest probability to be reached via emission from the lowest lying vibrational levels of A(1)Π. The work had a dual outcome: First, it explored consequences of different choices of the state-specific reference "Fermi-sea" space ("active" space), which is required for the construction and execution of the multiconfigurational "complete active space self-consistent field" calculations and the subsequent multi-reference CI calculations. In this context, comparisons with results on the weakly bound ground states of the homonuclear Be(2) and Mg(2) molecules were made. Second, it produced reliable data for the short- as well as the long-range parts of the potential energy curve (PEC). Such information is relevant to analyses concerning cold and ultra-cold Physics and Chemistry. For example, accurate fits to the X(1)Σ(+) PEC, which was computed to nano-Hartree accuracy, with account for basis-set-superposition error, produced the C(6) and C(8) dispersion coefficients as 364.3 ± 1.1 a.u. and 28 000 ± 500 a.u., respectively. The result for C(6) is in excellent agreement with that of Derevianko et al. [At. Data Nucl. Data Tables 96, 323 (2010)], (364 ± 4 a.u.), that was obtained in the framework of the theory of long-range interactions and many-body calculations on the constituent atoms. On the other hand, our result for C(8) differs from that of Standard and Certain [J. Chem. Phys. 83, 3002 (1985)] by about 7000 a.u.