The use of non-orthogonal orbitals allows the construction and use of more compact wave functions than offered by standard methods using orthogonal molecular orbitals; in particular, for molecules containing partly occupied atomic orbitals as present, for example, in transition metal complexes. With the purpose of developing efficient dynamic correlation methods, we discuss several new internal correlation methods employing a reference state containing non-orthogonal active orbitals. The non-orthogonal internally contracted perturbation theory approach is improved in several directions. The major improvements are the use of the Dyall Hamiltonian including two-electron interactions within the active space as the zero-order operator, the calculation of third-order energy-corrections, and the inclusion of excitations in the space of active orbitals. The latter improvement corrects for the use of an incomplete reference state. The improvements are tested for the nitrogen molecule and the challenging chromium dimer. The combined use of the improved zero-order Hamiltonian and the inclusion of active space excitations allow us to obtain potential curves for the chromium dimer that are close to those obtained using the larger complete active space reference wave function.

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非正交参考状态的动态相关：改进的微扰和变分方法

与使用正交分子轨道的标准方法相比，使用非正交轨道可以构建和使用更紧凑的波函数；特别是对于例如在过渡金属配合物中存在的含有部分占据的原子轨道的分子。为了开发有效的动态相关方法，我们讨论了几种新的内部相关方法，它们采用了包含非正交活动轨道的参考状态。非正交内部收缩摄动理论方法在几个方向上都得到了改进。主要的改进是使用Dyall哈密顿量，其中包括在活动空间中作为零阶算子的两电子相互作用，三阶能量校正的计算以及在活动轨道空间中的激发。后一种改进纠正了不完整参考状态的使用。测试了对氮分子和具有挑战性的铬二聚体的改进。结合使用改进的零阶哈密顿量和包含有效空间激发，可以使我们获得铬二聚体的电势曲线，该电势曲线接近于使用较大的完整有源空间参考波函数获得的电势曲线。