物理化学学报 >> 2000, Vol. 16 >> Issue (10): 912-919.doi: 10.3866/PKU.WHXB20001009

研究论文 上一篇    下一篇

精确模型化学中MPn微扰外推的一个问题

苏克和, 魏俊, 胡小玲, 岳红, 吕玲, 王育彬, 文振翼   

  1. 西北工业大学化工系 西安 710072|西北大学现代物理所 西安 710068
  • 收稿日期:2000-02-21 修回日期:2000-05-09 发布日期:2000-10-15
  • 通讯作者: 苏克和 E-mail:sukehe@nwpu.edu.cn

Single-reference M?ller-Plesset Perturbation Extrapolation Technique in the Accurate Model Chemistry is Unreliable for Non-equilibrium Molecules

Su Ke-He, Wei Jun, Hu Xiao-Ling, Yue Hong, Lv Ling, Wang Yu-Bin, Wen Zhen-Yi   

  1. Department of Chemical Engineering,Northwestern Polytechnical University,Xi'an 710072|Institute of Modern Physice,Northwest University,Xi'an Shanxi 710068
  • Received:2000-02-21 Revised:2000-05-09 Published:2000-10-15
  • Contact: Su Ke-He E-mail:sukehe@nwpu.edu.cn

摘要:

通过对 BH、 AlH、 BF和 AlF的高级别 ab initio势能曲线计算和理论分析 ,发现在分子的几何构型偏离平衡时 ,由于参考态组态不充分的问题 ,Hartree-Fock单参考态的 MPn微扰计算可能给出错误的能量值 .从而表明,目前的精确模型化学方法 (G1、 G2、 G3和 CBS系列 )中用 MPn微扰外推的办法拟合高级别 ab initio,对偏离平衡构型体系 (如动力学研究中的过渡态、中间体等 )的能量计算 ,在一般意义上是不可靠的 .但是 ,在相同的参考态上 ,即使 MPn微扰能已经发生错误 ,QCISD(T)、 CCSD(T)方法却能较好地描述分子解离过程的能量 ,且 QCISD(T)比 CCSD(T)更好 .因此 ,在单参考态理论框架内,发展以严格的 QCISD(T)方法为基础的模型化学方法 ,似更适用于反应动力学方面的研究 .

关键词: 单参考态, MPn微扰, 外推, 模型化学, 非平衡构型, 不可靠

Abstract:

Potential energy curves of BH(1Σ ) and AlH(1Σ ) molecules in their ground electronic states were calculated at RQCISD(T), UQCISD(T), RCCSD(T), UCCSD(T) and CASSCF (with four active electrons in five active orbitals) theoretical levels. Basis sets examined were 6-311+ G(3df,2p),G3 large ,aug-cc-pVDZ and aug-cc-pVTZ and the bond lengths tested were up to 1.0 nm. Potential energy curves of BF(1Σ ) and AlF(1Σ ) were also examined at MP4/aug-cc-pVDZ level. It was shown that the MPn (n=2- 4) curves for BH, AlH and BF were seriously in error. In accordance with CASSCF wavefunctions, the single Hartree-Fock reference state was insufficient to describe the electronic structure of those molecules. In this case, the Hartree-Fock energy gaps between the LUMO and HOMO orbitals for these molecules were decreased when the bond distances were increased. By examination of the molecular orbital integrals, it was found that the interaction (integral values) between HOMO and LUMO orbitals increased greatly. As in the case of second-order MP perturbation calculations, the "stronger interaction" and the "smaller energy gap" resulted in too large correlation energy corrections and therefore led to serious errors in the total electronic energy. This predicted clearly that the MPn extrapolation technique in the widely used G1,G2,G3 and CBS series accurate model chemistry was not reliable in general if the molecule is not at its equilibrium geometry, as what in the study of chemical reaction paths. However, QCISD(T) calculations reflected approximately the correct potential energy curves and more reliable dissociation limits. The maximum error,40.6 kJ• mol- 1,of dissociation energy at the bond length of 1.0 nm was found in AlH molecule by the G2 basis sets 6-311+ G(3df,2p).The CCSD(T) was also found less reliable than the QCISD(T) method in describing the dissociation energies. The same error was 101.9 kJ• mol- 1.All of the results in this work suggested the non-extrapolation G2(QCI) be more reliable a method in the energy calculations of non-equilibrium structure of molecules and be more valuable in further developments and modifications to the methods of accurate model chemistry.

Key words: Single reference state, Mller-Plesset perturbation, Extrapolation, Model chemistry, Non-equilibrium structure, Unreliable