A note on density matrix extrapolation and multiple solutions of the unrestricted Hartree–Fock equations

Unrestricted Hartree–Fock (UHF) SCF–MO calculations on the doublet reaction surface for the addition of methylidyne (CH) to ethylene (C₂H₄) using the standard extrapolation techniques of the GAUSSIAN 70 program show erratic behavior. On the other hand, the potential energy surface calculated without extrapolation of the density matrix and by using the final density matrix of a neighboring point as the initial guess for the density matrix for the new point gave a smooth potential curve without any kinks or erratic pattern. Therefore, the density extrapolation technique should be used with particular caution in UHF calculations.