Wu Qin, Cheng Chiao-Lun, Van Voorhis Troy
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2007 Oct 28;127(16):164119. doi: 10.1063/1.2800022.
Existing density functional theory (DFT) methods are typically very effective in capturing dynamic correlation, but run into difficulty treating near-degenerate systems where static correlation becomes important. In this work, we propose a configuration interaction (CI) method that allows one to use a multireference approach to treat static correlation but incorporates DFT's efficacy for the dynamic part as well. The new technique uses localized charge or spin states built by a constrained DFT approach to construct an active space in which the effective Hamiltonian matrix is built. These local configurations have significantly less static correlation compared to their delocalized counterparts and possess an essentially constant amount of self-interaction error. Thus their energies can be reliably calculated by DFT with existing functionals. Using a small number of local configurations as different references in the active space, a simple CI step is then able to recover the static correlation missing from the localized states. Practical issues of choosing configurations and adjusting constraint values are discussed, employing as examples the ground state dissociation curves of H(2) (+), H(2), and LiF. Excellent results are obtained for these curves at all interatomic distances, which is a strong indication that this method can be used to accurately describe bond breaking and forming processes.
现有的密度泛函理论（DFT）方法通常在捕捉动态关联方面非常有效，但在处理静态关联变得重要的近简并系统时会遇到困难。在这项工作中，我们提出了一种组态相互作用（CI）方法，该方法允许使用多参考方法来处理静态关联，同时也结合了DFT在动态部分的有效性。新技术使用由约束DFT方法构建的局域电荷或自旋态来构建一个有效哈密顿矩阵在其中构建的活性空间。与非局域对应物相比，这些局域组态的静态关联显著减少，并且具有基本恒定的自相互作用误差量。因此，它们的能量可以通过使用现有泛函的DFT可靠地计算出来。在活性空间中使用少量局域组态作为不同的参考，然后一个简单的CI步骤就能恢复局域态中缺失的静态关联。文中讨论了选择组态和调整约束值的实际问题，并以H(2) (+)、H(2)和LiF的基态解离曲线为例。在所有原子间距离处，这些曲线都获得了优异的结果，这有力地表明该方法可用于准确描述键的断裂和形成过程。
