Projected unrestricted Mo/ller–Plesset second-order energies

Abstract

A practical scheme for obtaining unrestricted Mo/ller–Plesset energies at second order (UMP2), with spin contamination removed from the wave function, is described and demonstrated. The algebra increases with the number of spin projections, and it appears to be practical to include only two spin projections. Full formulas are given, for which the cost is proportional to M3, where M is the number of basis functions. The results of these calculations give remarkably enhanced accuracy at fractionally extra cost. Four examples are included to demonstrate this: (i) the potential curve for the symmetric dissociation of H2O; (ii) the potential curve for the abstraction of H from CH4; (iii) the barrier for the addition of H to C2H4; and (iv) the electron affinity of CN. It is recommended that this procedure should be added at the end of all UMP2 calculations.