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通向凝聚态物理和量子化学的终极之路--密度矩阵重整化群，从矩阵乘积态到张量网态
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The key goal of quantum chemistry is the accurate calculation of geometrical and electronical ground state properties of molecules as well of their excited states. To this purpose, density functional theory is by far the most successful and flexible method. However, density functional theory can only be carried out by the use of an exchange-correlation functional which takes into account electronic correlation effects in the molecules. Unfortunately, this functional is only known approximately, and an important topic in quantum chemistry is the calculation of correlation effects in molecules („post-Hartree-Fock“ calculations). A (so-called) full configuration interaction (CI) calculation is limited to molecules with an extremely small number of orbitals due to the exponential explosion of quantum basis states; hence various approximate schemes to take into account subsets of quantum basis states have been proposed. A similar situation is encountered in the study of strongly correlated quantum systems in condensed matter physics: in the study of lattice models such as the Hubbard or Heisenberg models, which are considered to capture the essentials of low-dimensional quantum magnetism, high-temperature superconductivity and other novel quantum states, the question of identifying relevant subsets of quantum basis states has been at the forefront of research for quite some time. In the case of one-dimensional systems, the so-called density-matrix renormalization group method (DMRG) has emerged as the most powerful method to study correlation effects, both statically and dynamically. From an application point of view, it can be seen as an extension of exact diagonalization methods which are the counterpart of CI in physics. DMRG can therefore be used to extend the reach of CI in quantum chemistry, which has been successfully done by several groups worldwide. However, this is a complicated endeavour: whereas in physics the external one-dimensional lattice provides a natural ordering of sites (or orbitals), this is not the case in quantum chemistry, where the method must be optimized by a clever choice of orbital sets and ordering of orbitals on a pseudo-one-dimensional axis with long-ranged interactions. Nevertheless, impressive accuracies on the level of CI have been achieved. Recently, a thorough reformulation of DMRG in terms of so-called matrix product states (MPS) has shown a profound connection of this method to quantum information theory and revealed that it is only one special method in a much more general set of methods that can give variationally optimal results for much more complicated „lattice“ arrangements: in physics, these would be quantum states on two- or even three-dimensional lattices (so-called tensor network states), The basic idea of tensor network states is to approximate ground-state wave functions of strongly correlated systems by breaking down the complexity of the high dimensional coefficient tensor of a full configuration-interaction (FCI) wave function and the current hope is that these methods will shed light on some of the big outstanding questions in condensed matter physics. But again, there is a connection to quantum chemistry: these general networks of sites (or orbitals) are not restricted to regular lattices, but can be adapted to the complex arrangements and interactions between quantum chemistry orbitals. In the most naive approach, these arrangements would follow the expected geometrical structure of the molecule under study. However, this can be done more systematically by studying entanglement properties of quantum chemical states, as entanglement turns out to determine the efficiency and accuracy of these methods. Very little is known so far, but it is clear that the current restrictions of DMRG in quantum chemistry would be largely lifted due to the much more flexible setups allowed by tensor network states. [ Last edited by chrinide on 2011-5-7 at 09:26 ] |
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3楼2011-05-07 00:24:56
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如Garnet Chans所说，http://arxiv.org/abs/0711.1398 单组态为主的体系DMRG影响有限 'Conversely, the ansatz is inefficient for describing dynamic correlation, since this benefits from knowledge of the occupied and virtual spaces' [ Last edited by fichte on 2011-5-20 at 18:38 ] |
2楼2011-05-06 23:22:19
4楼2011-05-07 00:28:41
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几乎所有的多电子体系都是需要多组态多参考来精确的描述，单组态近似在某些时候的确很有效，这取决于我们需要得到体系多么精确的物理化学性质。 从量子场论（QFT）或者量子统计力学（QSM）的角度出发，多电子体系的关联能起源于电荷涨落和磁涨落（包括自旋涨落【SFT】和轨道涨落【OFT】）。例如，最近的研究直接表明高温超导体的内在机理和体系的强的自旋涨落密切相关，即便是精确的动力学平均场（DMFT）也不能很好(确切地说几乎不成功)的描述，DMFT包含了时间尺度的非局域量子涨落，但冻结了空间尺度的非局域量子涨落－长程库仑相互作用－电磁涨落。 因此，高温超导的理论研究方向之一就是 DMFT+SFT 经典的DMRG理论是一个基态理论，新的进展包括 TD-DMRG，动力学DMRG...... 其实即便只是考虑基态，0温，我们离终极之路也非常非常之遥远。 考虑有限温度－热涨落－体系的激发态－单电子高激发态－FCI failed－MQDT（多通道量子亏损理论）－双电子及多电子激发态－电离态－等离子体......这些基本上属于无人敢碰的看似简单的实则具有极其深刻的物理意义的艰难课题，数十年基本没有多大进展。 引用一个古老的炼金术士格言: 未知必须借由更深的未知来获悉，隐晦必须借由更深的隐晦来明了 [ Last edited by chrinide on 2011-5-13 at 08:19 ] |
5楼2011-05-12 23:05:21