Elimination of singularities in molecular orbital derivatives: minimum orbital-deformation (MOD) method

We present the minimum orbital-deformation (MOD) method which effectively solves the problem intrinsic to the local approach for calculating the electron correlations. It eliminates the discontinuities in potential energy surfaces (PES) and properties, and hence the singularities of their derivatives which can occur when the localized MOs are used. The present method defines smooth invariant transformation of SCF MOs along the geometrical change, which is similar to the quasidiabatization. We demonstrate the performance of the MOD method for the ground state PES of benzene, for which artificial errors were reported in the analytical energy gradients of the local MP2.