卟吩结构的HARTREE-FORCK和密度泛函研究比较

Comparative studies on the structures of porphyrin(H_2P),using hartree-fock and density functional theory methods

采用传统的Hartree—Fock（HF）从头算和密度泛函中的B3LYP和B3PW91方法，选择了不同基组水平（STO-3G，3-21G和6-31G^＊）对卟吩结构进行了全几何优化，并对得到的结果相互进行了比较，同时与实验数据进行了对照。结果表明，对于H2P以及具有类似结构的卟啉衍生物，传统的HF方法得到的几何构型以及相关性质（核磁共振谱，NMR）并不可靠。这种体系中电子相关能是不可忽略的。在B3LYP水平上选用较小的基组3-21G得到的结果与中等基组6-31G^＊差别不大。但是最小基组STO-3G得到的优化构型却是一个鞍点结构。

The molecular structure of Free-Base Porphyrin（ H2P） has been fully optimized using the Density Functional Theory （DFT） B3LYP and B3PW91 methods, as well as ab initio Hartree-Fock method. Various basis sets STO-3G, 3-21G and 6-31G^＊ have been employed. The results obtained at various levels are discussed and compared with each other and with the available experimental data. It was shown that calculations performed at the Hartree-Fock level cannot produce a reliable geometry and related properties such as the Nuclear Magnetic Resonance （NMR） of H2P and similar porphyrin-based systems. Electron correlation must be included in the calculations. The basis set has comparatively less effect on the calculated results. The results derived at the B3LYP level using the smaller 3-21G basis set were very close to those produced using the medium 6-31 G^＊ basis set. However, the optimized structure obtained from the smallest basis set, STO-3G, was a saddle type.