A configuration interaction study of the four lowest 1∑+ states of the LiH molecule

A configuration interaction study was completed on the ¹∑⁺ states of the LiH molecule using a nonorthogonal one-electron basis in elliptical coordinates. A few wave functions with highly optimized parameters were obtained for the X¹∑⁺ and A¹∑⁺ states and combined to construct a larger wave function which gave improved results for both states over a wide range of R values. The third and fourth roots are also reported since the wave function is extensive enough to give good upper bounds for the two states. Calculations were completed for 34 values R in the range 1 ≤ R ≤ 10 bohr (b). The calculated X¹∑⁺ curve has a minimum at Re = 3.060b (3.015b), with E(Re) = ?8.0556 Hartree (H) (?8.0704), μ(Re) = ?5.89 debye (d) (?5.88d) and μ/(R dμ/dR)|_Re = 1.75 (1.80 ± 0.3). For the A¹∑⁺ state, Re = 4.928b (4.906b), E(Re) = ?7.9372H(?7.9496H), μ(Re) = +3.96d and μ/(R dμ/dR)|_Re = ?-0.471. The values in parentheses are experimental results for comparison. The numerical vibrational and rotational analysis agreed well with experiment for both states. The A¹∑⁺ state exhibited a pronounced negative anharmonicity. Both states show strong interaction of three zeroth-order configurations, the nature of which changes considerably with R. The thus far unobserved second excited state has two minima, a metastable one at R = 3.70b and another at R ≈ 10.00b. The third excited state also appears to have a minimum at R ≈ 7.00b.