ORMAS-SCF study of silicon (100) surface clusters.

An occupation restricted multiple active space (ORMAS) study of clusters that represent the silicon(100) surface (up to nine surface dimers) is discussed. The accuracy of three different active orbital ORMAS partition schemes for Si(100) surface clusters are compared. In addition to ORMAS-SCF calculations, generalized valence bond-perfect pairing (GVB-PP) properties are generated for comparison purposes. The ability of ORMAS to generate a reliable multiconfigurational zeroth-order wave function is systematically tested and when possible is compared to the full complete active space self-consistent field (CASSCF) method. This provides good benchmarks for the accuracy of ORMAS compared to CASSCF. It is demonstrated that ORMAS consistently provides a high degree of accuracy with a significantly reduced computational effort relative to a CASSCF calculation. For the largest cluster, for which a full CASSCF calculation is not possible, ORMAS predicts that the Si(100) surface dimers are symmetric.