Are both symmetric and buckled dimers on Si(100) minima? Density functional and multireference perturbation theory calculations

Abstract

We report a spin-unrestricted density functional theory(DFT) solution at the symmetric dimer structure for cluster models of Si(100). With this solution, it is shown that the symmetric structure is a minimum on the DFTpotential energy surface, although higher in energy than the buckled structure. In restricted DFT calculations the symmetric structure is a saddle point connecting the two buckled minima. To further assess the effects of electron correlation on the relative energies of symmetric versus buckled dimers on Si(100), multireference second order perturbation theory (MRMP2) calculations are performed on these DFT optimized minima. The symmetric structure is predicted to be lower in energy than the buckled structure via MRMP2, while the reverse order is found by DFT. The implications for recent experimental interpretations are discussed. KeywordsDensity functional theory, Perturbation theory, Electron correlation calculations, Potential energy surfaces Disciplines Chemistry CommentsThe following article appeared in Journal of Chemical Physics 119 (2003) We report a spin-unrestricted density functional theory ͑DFT͒ solution at the symmetric dimer structure for cluster models of Si͑100͒. With this solution, it is shown that the symmetric structure is a minimum on the DFT potential energy surface, although higher in energy than the buckled structure. In restricted DFT calculations the symmetric structure is a saddle point connecting the two buckled minima. To further assess the effects of electron correlation on the relative energies of symmetric versus buckled dimers on Si͑100͒, multireference second order perturbation theory ͑MRMP2͒ calculations are performed on these DFT optimized minima. The symmetric structure is predicted to be lower in energy than the buckled structure via MRMP2, while the reverse order is found by DFT. The implications for recent experimental interpretations are discussed.

On the structure of Si(100) surface: Importance of higher order correlations for buckled dimer

“…Our earlier study of the silicon dimers found the UB3LYP solution to indeed exist and be lower in energy than the B3LYP solution, the spinunrestricted solution was, therefore, preferred in a variational sense. 25 However, in the present study we find that using spin-restricted Kohn-Sham orbitals yields a lower XYGJ-OS energy when additionally considering the perturbative correlation effects. The same behavior was observed for other DHDFs considered here.…”

“…Interestingly, a similar trend was observed in the DFT calculations for two, three, and four dimers. 25 The CASPT2 calculations show an opposite trend. We also note that the MRCI (+Q) calculations on (U)B3LYP geometries yield almost same results, showing a weak geometry dependence between (U)B3LYP and XYGJ-OS in the present case.…”

“…We also included the single-point DHDF/6-311+G(3df,2p) energy calculations on the previous (U)B3LYP/6-31G(d) geometries. 25 Positive value means that the buckled structure is lower in energy. Like most existing DFT results, DHDFs favor the buckled structure.…”

“…14 In theoretical literature, generally, Hartree-Fock (HF), density functional theory (DFT), and quantum Monte Carlo (QMC) predicted the buckled structure to be the lowest energy minimum, while the second order perturbation theory (MP2), multi-configurational self-consistent field (MC-SCF), multi-reference configuration interaction (MRCI), and multi-reference second order perturbation theory (MRMP2) optimizations suggested the symmetric structure to be the global minimum. [18][19][20][21][22][23][24][25][26] The CCSD(T) and MRMP2 geometry optimizations for one dimer Si-cluster (Si 9 H 12 ) have predicted its structure to be symmetric. 22 A recent study using the unrestricted density functional theory (UDFT) with B3LYP functional showed that both symmetric and buckled minima exist, although the energy of buckled configuration was marginally lower than that of the symmetric by 1.40 kcal/mol per dimer for a three-dimer cluster model.…”

Dalton Discussion 11: The renaissance of main group chemistry

“…While clearly needed for dissociation, there are also some cases where it is known that significantly different stable structures can be obtained in DFT when spin-polarization is allowed. 24 …”