Most multi-reference methods that are used to compute accurate ground and excited states use the ground state Hartree-Fock wavefunction to build the excited state determinants. This is done for example in the Complete Active Space Second Order Perturbation theory (CASPT2) method. Our method uses the ?SCF procedure to compute non-aufbau excited state determinants. The ground state and excited states are then all solutions to the Hartree-Fock Hamiltonian and are therefore treated with the same level of theory from the start. We can then add a second order M�ller Plesset (MP2) perturbation theory correction to each reference determinant. Finally we diagonalize the Hamiltonian in this new bases to get the final ground and excited states. Our new method yields similar accuracy to the CASPT2 method but with a smaller active space.

Treating ground and excited states equally from the start: A new multi-reference configuration interaction method with second order MØller-Plesset perturbation theory

245th National Meeting (2013)

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