Block2: A comprehensive open source framework to develop and apply state-of-the-art DMRG algorithms in electronic structure and beyond.

block2 is an open source framework to implement and perform density matrix renormalization group and matrix product state algorithms. Out-of-the-box it supports the eigenstate, time-dependent, response, and finite-temperature algorithms. In addition, it carries special optimizations for ab initio electronic structure Hamiltonians and implements many quantum chemistry extensions to the density matrix renormalization group, such as dynamical correlation theories. The code is designed with an emphasis on flexibility, extensibility, and efficiency and to support integration with external numerical packages. Here, we explain the design principles and currently supported features and present numerical examples in a range of applications.

Conservation laws in coupled cluster dynamics at finite temperature.

We extend the finite-temperature Keldysh non-equilibrium coupled cluster theory (Keldysh-CC) [A. F. White and G. K.-L. Chan, J. Chem. Theory Comput. 15, 6137-6253 (2019)] to include a time-dependent orbital basis. When chosen to minimize the action, such a basis restores local and global conservation laws (Ehrenfest's theorem) for all one-particle properties while remaining energy conserving for time-independent Hamiltonians. We present the time-dependent Keldysh orbital-optimized coupled cluster doubles method in analogy with the formalism for zero-temperature dynamics, extended to finite temperatures through the time-dependent action on the Keldysh contour. To demonstrate the conservation property and understand the numerical performance of the method, we apply it to several problems of non-equilibrium finite-temperature dynamics: a 1D Hubbard model with a time-dependent Peierls phase, laser driving of molecular H2, driven dynamics in warm-dense silicon, and transport in the single impurity Anderson model.

Simulating X-ray Absorption Spectra with Linear-Response Density Cumulant Theory.

We present a new approach for simulating X-ray absorption spectra based on linear-response density cumulant theory (LR-DCT) [ Copan , A. V. ; Sokolov , A. Yu. J. Chem. Theory Comput. 2018 , 14 , 4097 - 4108 ]. Our new method combines the LR-ODC-12 formulation of LR-DCT with core-valence separation approximation (CVS) that allows us to efficiently access high-energy core-excited states. We describe our computer implementation of the CVS-approximated LR-ODC-12 method (CVS-ODC-12) and benchmark its performance by comparing simulated X-ray absorption spectra to those obtained from experiment for several small molecules. Our results demonstrate that the CVS-ODC-12 method shows good agreement with experiment for relative spacings between transitions and their intensities, but the excitation energies are systematically overestimated. When compared to results from excited-state coupled cluster methods with single and double excitations, the CVS-ODC-12 method shows a similar performance for intensities and peak separations, while coupled cluster spectra are less shifted, relative to experiment. An important advantage of CVS-ODC-12 is that its excitation energies are computed by diagonalizing a Hermitian matrix, which enables efficient computation of transition intensities.

A Hexavalent Basket for Bottom-Up Construction of Functional Soft Materials and Polyvalent Drugs through a "Click" Reaction.

Inspired by polyvalency and its prevalence in nature, we developed an efficient synthetic route for accessing a large variety of multivalent and dual-cavity baskets from inexpensive and abundant starting materials. First, the cycloaddition of vinyl acetate to anthracene was optimized to, upon hydrolysis, give dibenzobarrelene derivative 6, which after five functional group transformations and then cyclotrimerization gave heptiptycene dodecaester 4 in an overall 17 % yield. Following that, compound 4 was converted into D3h symmetric 1, composed of two fused cavitands each holding three terminal alkynes at the rim for conjugation to functional molecules using the highly efficient CuAAC reaction. To survey the reactivity of hexavalent 1, we "clicked" 2-acetamido-2-deoxy-ß-d-glucopyranosyl azide 3,4,6-triacetate (carbohydrate), methoxypolyethylene glycol azide (PEG, Mn =2000; polymer) and benzyl azide (aromatic) to obtain hexavalent conjugates 12-14 in 50-79 % yields. In summary, dual-cavity 1 is an accessible, structurally-unique and hexavalent host that can be "clicked" to a variety of functional molecules for (a) combinatorial lead identification of drugs, (b) preparation of hierarchical soft materials and (c) design of selective chemosensors, scavengers, or supramolecular catalysts.