Model systems for exploring electron correlation effects in the buckling of SiSi dimers on the Si(100) surface

Si2H4 and Si7H8 cluster models are constructed for studying the role of high-order electron correlation effects on the buckling of SiSi dimers on the Si(100) surface. CASPT3, multi-reference coupled-pair functional, and the diffusion Monte Carlo methods are used to examine whether correlation effects not recovered in CASPT2 calculations are important for the buckling of the dimers. The calculations show that such high-order correlation effects are indeed important for determining the relative stability of the buckled and unbuckled structures.