Bessac Fabienne, Hoyau Sophie, Maynau Daniel
Laboratoire de Physique Quantique, Unité Mixte de Recherche (UMR) 5626 du Centre National de la Recherche Scientifique (CNRS), Institut de Recherche sur les Systemes Atomiques et Moléculaires Complexes (IRSAMC), Université Paul Sabatier, Toulouse, France.
J Chem Phys. 2005 Sep 8;123(10):104105. doi: 10.1063/1.2008215.
Thanks to the use of localized orbitals and the subsequent possibility of neglecting long-range interactions, the linear-scaling methods have allowed to treat large systems at ab initio level. However, the limitation of the number of active orbitals in a complete active space self consistent-field (CASSCF) calculation remains unchanged. The method presented in this paper suggests to divide the system into fragments containing only a small number of active orbitals. Starting from a guess wave function, each orbital is optimized in its corresponding fragment, in the presence of the other fragments. Once all the fragments have been treated, a new set of orbitals is obtained. The process is iterated until convergence. At the end of the calculation, a set of active orbitals is obtained, which is close to the exact CASSCF solution, and an accurate CASSCF energy can be estimated.
由于使用了定域轨道以及随后忽略长程相互作用的可能性，线性标度方法使得在从头算水平上处理大体系成为可能。然而，完全活性空间自洽场（CASSCF）计算中活性轨道数量的限制仍然不变。本文提出的方法建议将体系划分为仅包含少量活性轨道的片段。从一个猜测波函数开始，在存在其他片段的情况下，每个轨道在其相应的片段中进行优化。一旦所有片段都得到处理，就会获得一组新的轨道。该过程反复进行直至收敛。在计算结束时，会得到一组接近精确CASSCF解的活性轨道，并且可以估计出准确的CASSCF能量。
