蒙特卡罗组态相互作用的发展：自然轨道和二级微扰理论
Development of Monte Carlo configuration interaction: Natural orbitals and second-order perturbation theory
摘要：
研究了近似自然轨道作为改进蒙特卡罗组态相互作用（MCCI）计算的一种方法。我们介绍了一种方法来近似自然轨道MCCI和测试这些和近似自然轨道MP2和QCISD在MCCI计算的单点能量。的效率和准确性的近似自然轨道的MCCI势曲线计算的双氢解离的水，一氧化碳的解离和氮分子的解离，然后被认为是在比较与标准MCCI时，使用全组态相互作用作为基准。我们还使用该方法来产生水的aug-cc-pVTZ基础上的电位曲线。提出了一种新的方法来量化的潜力曲线的准确性，考虑到所有的点，曲线可以移动一个常数。我们采用了二阶微扰方法来处理MCCI（MCCIPT 2），提高了该方法去除重复态的效率。MCCIPT 2是测试在使用斯莱特决定簇和配置态函数的氮的解离的电位曲线的计算。
英文摘要：
Approximate natural orbitals are investigated as a way to improve a Monte Carlo configuration interaction (MCCI) calculation. We introduce a way to approximate the natural orbitals in MCCI and test these and approximate natural orbitals from MP2 and QCISD in MCCI calculations of single-point energies. The efficiency and accuracy of approximate natural orbitals in MCCI potential curve calculations for the double hydrogen dissociation of water, the dissociation of carbon monoxide and the dissociation of the nitrogen molecule are then considered in comparison with standard MCCI when using full configuration interaction as a benchmark. We also use the method to produce a potential curve for water in an aug-cc-pVTZ basis. A new way to quantify the accuracy of a potential curve is put forward that takes into account all of the points and that the curve can be shifted by a constant. We adapt a second-order perturbation scheme to work with MCCI (MCCIPT2) and improve the efficiency of the removal of duplicate states in the method. MCCIPT2 is tested in the calculation of a potential curve for the dissociation of nitrogen using both Slater determinants and configuration state functions.
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