Ab-initio DMRG and Canonical Transformation Theories of Electronic Structure

Steven R. White

Takeshi Yanai, Yuki Kurashige, Eric Neuscamman +1 more

The canonical transformation (CT) theory has been developed as a multireference electronic structure method to compute high-level dynamic correlation on top of a large active space reference treated with the ab initio density matrix renormalization group method. This article describes a parallelized algorithm and implementation of the CT theory to handle large computational demands of the CT calculation, which has the same scaling as the coupled cluster singles and doubles theory. To stabilize the iterative solution of the CT method, a modification to the CT amplitude equation is introduced with the inclusion of a level shift parameter. The level-shifted condition has been found to effectively remove a type of intruder state that arises in the linear equations of CT and to address the discontinuity problems in the potential energy curves observed in the previous CT studies.

Johannes Hachmann

This thesis is concerned with two topics from the field of computational electronic structure theory. In the first part we will describe our contributions to the ab initio density matrix renormalization group (DMRG) methodology in quantum chemistry, in particular our quadratically scaling local algorithm. We will present applications aimed towards new insights into the physics of conjugated π-electron systems as found, e.g., in organic electronic materials. The second part of this thesis covers our computational study of 3d-M(smif)2 complexes synthesized and characterized in the Wolczanski Group. These compounds exhibit unusual electronic structure phenomena which we address from a theoretical perspective.

Yifan Cheng, Zhaoxuan Xie, Haibo Ma

The ab initio density matrix renormalization group (DMRG) method has been well-established and has become one of the most accurate numerical methods for the precise electronic structure solution of large active spaces. In the past few years, to capture the missing dynamic correlation, various post-DMRG approaches have been proposed through the combination of DMRG and multireference quantum chemical methods or density functional theory. With this in mind, this work provides a brief overview of ab initio DMRG principles and the new developments within post-DMRG methods. For clarity, post-DMRG methods are classified into two main categories depending on whether high-order n -electron reduced density matrices are used, and their merits and disadvantages are properly discussed. Finally, we conclude by discussing unsolved bottlenecks and giving development perspectives of post-DMRG approaches, which are expected to yield quantitative descriptions of complex electronic structures in large strongly correlated molecules and materials.

Eugene S. Kryachko, Eduardo V. Ludeña