A two-step uncontracted determinantal effective Hamiltonian-based SO-CI method

We present a new two-step uncontracted spin-orbit configuration interaction (CI) method which automatically accounts for spin-orbit polarization effec ts on multiconfigurational wave functions by selecting the single excitatio ns having a significant spin-orbit interaction with a chosen determinantal reference space. This approach is in the line of a conventional two-step me thod, as a sophisticated correlation treatment in a scalar relativistic app roximation is carried out in the first step. In the second step, we define a model space which includes a set of reference configurations able to repr esent all the wanted states along with singly excited configurations select ed with the spin-orbit (SO) operator. We then exploit the first-step calcul ation in order to include correlation effects via an effective Hamiltonian technique and diagonalize the full matrix on the determinantal basis. The m ethod combines the advantages of both one-step and conventional two-step SO -CI methods; it intends to treat efficiently the cases where both relativit y and extended CI treatments are needed. The new code EPCISO is tested on t he spin-orbit splitting of the P-2 electronic ground state of the thallium atom which, in spite of its very simple electronic structure is a well-know n difficult case study for SO-CI methods. The EPCISO code yields results in excellent agreement with the experimental splitting value; they are compar ed to those obtained using the conventional two-step CIPSO code. (C) 2000 A merican Institute of Physics. [S0021-9606(00)30625-0].