Abstract Ab initio calculations of the potential energy surface of the system N 2Li + are presented both in SCF approximation and with electron correlation. The interaction energy has a minimum of about 0.56 eV for the linear configuration at a distance R = 5.0 a 0 from Li + to the center of mass of N 2, and a saddle point with a depth of 0.20 eV for the perpendicular arrangement at R = 4.5 a 0, both in SCF approximation. Electron correlation does not alter the location of the extrema markedly, but increases the depth of the minimum by about 0.005 eV, raising the saddle point by about the same amount. The analysis of the angular components of the interaction energy shows that V 4 and V 6 have short range behaviour and vanish for R > 5 a 0 whereas V 0 and V 2 are of long range with minima of 0.25–0.32 eV at R = 5.0 a 0. The change of the N 2 equilibrium force constant due to the approach of Li + is discussed as well in SCF approximation as with correlation.