Potential energy surfaces for the 1 (4)A('), 2 (4)A(') 1 (4)A(") and 2 (4)A(") states of Li(3).

Global potential energy surfaces for the 1 (4)A('), 2 (4)A('), 1 (4)A("), and 2 (4)A(") spin-aligned states of Li(3) are constructed as sums of a diatomics-in-molecules (DIM) term plus a three-body term. The DIM model, using a large basis set of 15 (4)A(") and 22 (4)A(') states, is used to obtain a "mixed-pairwise additive" contribution to the potential. A global fit of the three-body terms conserves the accuracy of the ab initio points of a full configuration-interaction calculation. The resulting fit accurately describes conical intersections for both the 1 (4)A(') and 2 (4)A(') surfaces with a root-mean-square (rms) deviation of 5.4x10(-5) hartree in D(infinityh) geometries and 1.2x10(-4) hartree in C(infinityv) geometries. The global fit appears to be quantitatively correct with a rms deviation of 1.8x10(-4)hartree for 1 (4)A('), 9.2x10(-4) hartree for 2 (4)A('), 2.5x10(-4) hartree for 1 (4)A("), and 5.1x10(-4) hartree for 2 (4)A("). A possible diabolic conical intersection, also called an accidental degeneracy, in C(2v) geometries, indicating a seam of conical intersections in C(s) geometries, is also found in ab initio calculations for A(2) states. As shown in this example, the DIM procedure can be optimized to describe the geometric phase and nonadiabatic effects in multisurface potentials.