In most nuclear many-body methods, observables are calculated using many-body wave functions explicitly. The variational two-particle reduced density matrix method is one of the few exceptions to the rule. Ground-state energies of both closed-shell and open-shell nuclear systems can indeed be evaluated by minimizing a constrained linear functional of the two-particle reduced density matrix. However, it has virtually never been used in nuclear theory, because nuclear ground states were found to be well overbound, contrary to those of atoms and molecules. Consequently, we introduced new constraints in the nuclear variational two-particle reduced density matrix method, developed recently for atomic and molecular systems. Our calculations then show that this approach can provide a proper description of nuclear systems where only valence neutrons are included. For the nuclear systems where both neutrons and protons are active, however, the energies obtained with the variational two-particle reduced density matrix method are still overbound. The possible reasons for the noticed discrepancies and solutions to this problem will be discussed.

中文翻译：

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核系统变分二粒子密度矩阵的再研究

在大多数核多体方法中，可观察量是明确使用多体波函数计算的。变分两粒子约简密度矩阵方法是该规则的少数例外之一。闭壳和开壳核系统的基态能量确实可以通过最小化双粒子降低密度矩阵的约束线性函数来评估。然而，它实际上从未在核理论中使用过，因为发现核基态很好地超界，与原子和分子的基态相反。因此，我们在最近为原子和分子系统开发的核变分两粒子约化密度矩阵方法中引入了新的约束。然后我们的计算表明，这种方法可以提供对仅包含价中子的核系统的正确描述。然而，对于中子和质子都活跃的核系统，用变分双粒子约简密度矩阵方法获得的能量仍然是超限的。将讨论引起注意的差异的可能原因以及此问题的解决方案。