Size consistent multireference single and double excitation configuration interaction calculations. The multireference coupled electron-pair approximation

A new size consistent extension to the multi reference configuration interaction method is described. The method termed multireference coupled electron pair approximation (MRCEPA) is akin to a multireference CEPA(0) approach, though nonlinear terms do receive separate attention. We show the performance of the approach in some model systems as well as in an application to calculation of ground and excited ππ* states of ethylene.