Local weak-pairs pseudospectral multireference configuration interaction

A reduced scaling MRSDCI method that simultaneously takes advantage of the pseudospectral and weak-pairs approximation is presented. This method offers important advantages for the development of reduced scaling configuration interaction (CI) algorithms, the most important of which are the separation of the spin and spatial parts of the wave function and on-the-fly calculation of integral coupling coefficients. It is shown that results comparable to those obtained using a fully analytic method can be obtained at a significantly reduced cost.