Structures of ground states of Li₂, LiS and Li₂S molecules are investigated using density function theory B3LYP method and 6-311++g(d,p) basis set. It shows that their electronic states are X¹Σ_g⁺, X²Π_g and X¹Σ_g⁺. Murrell-Sorbie potential functions with 4 parameters for LiS and Li₂ molecules are obtained with nonlinear curve fitting method. Spectral parameters and force constants are studied with Murrel-Sobie potential functions. Analytical potential surface of singlet Li₂S is obtained with many-body expansion theory. Rotary contour map, stretching contour map and rotary stretching contour map are reconstructed with analytical potential surface. Static characters of ground singlet Li₂S are repeated exactly through these maps. They are equilibrium geometric structure, lowest energy and reasonable chemical reaction channels, respectively. Stretching vibrational contours of potential energy surface show that reaction Li+S+Li→Li₂S is a no barrier reaction channel. Reaction channel of S atom attacking Li₂ molecule has a transition state. Channel of Li atom attacking LiS molecule has a transition state.