The graphical unitary group approach to the electron correlation problem. Methods and preliminary applications

Recent theoretical research by Paldus and by Shavitt has strongly suggested that the unitary group approach to the many body problem may be useful in molecular electronic problems. The graphical unitary group approach (GUGA) has now been developed into an extraordinarily powerful theoretical method. The theoretical/methodological contributions made here include a solution of the upper walk problem, the restriction of configuration space employed to the multireference interacting space, and the restructuring of the Hamiltonian in terms of loop types. Several test calculations are examined in detail to illustrate the unique features of the method. For large general multireference configuration interaction (CI) problems, computation times are typically only 15% of those reported using state-of-the-art conventional techniques. Finally, these methods are applied to the vertical electronic spectrum of ketene, and excellent agreement with experiment is found.