Rintelman Jamie M, Gordon Mark S, Fletcher Graham D, Ivanic Joseph
Department of Chemistry, Iowa State University and Ames Laboratory, Ames, Iowa 50011, USA.
J Chem Phys. 2006 Jan 21;124(3):034303. doi: 10.1063/1.2140687.
The three known lowest-energy isomers of SiC(3), two cyclic singlets (2s and 3s) and a linear triplet (1t), have been reinvestigated using multireference second-order perturbation theory (MRPT2). The dependence of the relative energies of the isomers upon the quality of the basis sets and the sizes of the reference active spaces is explored. When using a complete-active-space self-consistent-field reference wave function with 12 electrons in 11 orbitals [CASSCF (12, 11)] together with basis sets that increase in size up to the correlation-consistent polarized core-valence quadruple zeta basis set (cc-pCVQZ), the MRPT2 method consistently predicts the linear triplet to be the most stable isomer. A new parallel direct determinant MRPT2 code has been used to systematically explore reference spaces that vary in size from CASSCF (8,8) to full optimized reaction space [FORS or CASSCF (16,16)] with the cc-pCVQZ basis. It is found that the relative energies of the isomers change substantially as the active space is increased. At the best level of theory, MRPT2 with a full valence FORS reference, the 2s isomer is predicted to be more stable than 3s and 1t by 4.7 and 2.2 kcal/mol, respectively.
利用多参考二阶微扰理论（MRPT2）对已知的碳化硅（SiC(3)）的三种能量最低的异构体，即两个环状单重态（2s和3s）和一个线性三重态（1t）进行了重新研究。探讨了异构体相对能量对基组质量和参考活性空间大小的依赖性。当使用具有11个轨道中12个电子的完全活性空间自洽场参考波函数[完全活性空间自洽场（CASSCF(12, 11)）]以及尺寸增大至相关一致极化芯价四重zeta基组（cc-pCVQZ）的基组时，MRPT2方法一致预测线性三重态是最稳定的异构体。一种新的并行直接行列式MRPT2代码已被用于系统地探索参考空间，其大小从CASSCF(8,8)到完全优化反应空间[FORS或CASSCF(16,16)]，使用cc-pCVQZ基组。结果发现，随着活性空间的增加，异构体的相对能量发生了显著变化。在最佳理论水平，即具有全价FORS参考的MRPT2下，预测2s异构体分别比3s和1t稳定4.7和2.2千卡/摩尔。
