An efficient variational principle for the direct optimization of excited states

We present a variational function that targets excited states directly based on their position in the energy spectrum, along with a Monte Carlo method for its evaluation and minimization whose cost scales polynomially for a wide class of approximate wave functions. Being compatible with both real and Fock space and open and periodic boundary conditions, the method has the potential to impact many areas of chemistry, physics, and materials science. Initial tests on doubly excited states show that using this method, the Hilbert space Jastrow antisymmetric geminal power ansatz can deliver order-of-magnitude improvements in accuracy relative to equation of motion coupled cluster theory, while a very modest real space multi-Slater Jastrow expansion can achieve accuracies within 0.1 eV of the best theoretical benchmarks for the carbon dimer.Comment: 6 pages, 4 figure

Variational Excitations in Real Solids: Optical Gaps and Insights into Many-Body Perturbation Theory

We present an approach to studying optical band gaps in real solids in which quantum Monte Carlo methods allow for the application of a rigorous variational principle to both ground and excited state wave functions. In tests that include small, medium, and large band gap materials, optical gaps are predicted with a mean-absolute-deviation of 3.5% against experiment, less than half the equivalent errors for typical many-body perturbation theories. The approach is designed to be insensitive to the choice of density functional, a property we exploit in order to provide insight into how far different functionals are from satisfying the assumptions of many body perturbation theory. We explore this question most deeply in the challenging case of ZnO, where we show that although many commonly used functionals have shortcomings, there does exist a one particle basis in which perturbation theory's zeroth order picture is sound. Insights of this nature should be useful in guiding the future application and improvement of these widely used techniques.Comment: 8 pages, 5 figures, 2 table

Density Functional Extension to Excited-State Mean-Field Theory.

We investigate an extension of excited-state mean-field theory in which the energy expression is augmented with density functional components in an effort to include the effects of weak electron correlations. The approach remains variational and entirely time independent, allowing it to avoid some of the difficulties associated with linear response and the adiabatic approximation. In particular, all of the electrons' orbitals are relaxed state specifically, and there is no reliance on Kohn-Sham orbital energy differences, both of which are important features in the context of charge transfer. Preliminary testing shows clear advantages for single-component charge transfer states, but the method, at least in its current form, is less reliable for states in which multiple particle-hole transitions contribute significantly

A Blocked Linear Method for Optimizing Large Parameter Sets in Variational Monte Carlo

We present a modification to variational Monte Carlo's linear method optimization scheme that addresses a critical memory bottleneck while maintaining compatibility with both the traditional ground state variational principle and our recently-introduced variational principle for excited states. For wave function ansatzes with tens of thousands of variables, our modification reduces the required memory per parallel process from tens of gigabytes to hundreds of megabytes, making the methodology a much better fit for modern supercomputer architectures in which data communication and per-process memory consumption are primary concerns. We verify the efficacy of the new optimization scheme in small molecule tests involving both the Hilbert space Jastrow antisymmetric geminal power ansatz and real space multi-Slater Jastrow expansions. Satisfied with its performance, we have added the optimizer to the QMCPACK software package, with which we demonstrate on a hydrogen ring a prototype approach for making systematically convergent, non-perturbative predictions of Mott-insulators' optical band gaps.Comment: 9 pages, 3 tables, 4 figure

The Impact of the COVID-19 on Online Food Delivery Service: Evidence from China

The COVID-19 has had a profound effect on society as a whole. To examine the effect of the COVID-19 on online food delivery services, we collected sales data from a large online food delivery platform in 195 Chinese cities from November 2019 to July 2020. Interrupted time series analysis and time-varying difference-in-difference methods were used to estimate the impact of the COVID-19 and city lockdown policies on online food delivery services. The COVID-19 had a considerable negative effect on the online food delivery services. Lockdown policies caused further disruptions. As the pandemic and lockdown policies ended, the negative impacts dissipated. This finding reflected digital channels’ resilience to the catering industry during the pandemic and helped it withstand its impact. There were significant differences among urban characteristics. The government can formulate relevant policies to deal with potential public health risks in the future based on these findings

A Variational Principle for Modeling Electronic Excitations in Gas and Condensed Phase

Accurate modeling of electronic excited states is one of the most important and challenging problems in electronic structure theory. This thesis focuses on a recently developed excited state variational principle and its applications in gas and condensed phase. In contrast to the widely used excited states method such as linear response (LR) and many-body perturbation theory (MBPT), which find excited states by perturbing around the ground state wave function or a zeroth order particle-hole excitation picture, the new excited state variational principle directly targets excited states with the full flexibility of an approximate wave function ansatz. Due to its non-perturbative nature, this method offers balanced and systematically improvable descriptions to excited states. We will also discuss the efficient implementation of the new excited state variational principle through variational Monte Carlo and the Linear Method optimization algorithm.The new excited state variational principle is applied to predict both the excitation energies of low lying excited states in small molecules and optical gaps in solids. In molecules, the new method yields order-of-magnitude of improvements over the state-of-art excited state methods based on LR theory in double excitations. In solids, not only is the new method demonstrated to be more accurate than the commonly used MBPT method, but it could also be used to analyze and provide insights into MBPT. In order to further extend the method’s applicability, we introduce a modified optimization method that addresses a fatal memory bottleneck in the original algorithm. With only minor lose in accuracy, the modified algorithm reduces the required memory per parallel process from tens of gigabytes to hundreds of megabytes. With the aid of the new optimization method, we show that the new excited state variational principle could systematically converge the excitation energy in a strongly correlated, Mott-insulating hydrogen ring with respect to increasing flexibility in the wave function ansatzes

Magnetic Nanoparticle-Based Hyperthermia for Head & Neck Cancer in Mouse Models

In this study, magnetic iron oxide nanoparticle induced hyperthermia is applied for treatment of head and neck cancer using a mouse xenograft model of human head and neck cancer (Tu212 cell line). A hyperthermia system for heating iron oxide nanoparticles was developed by using alternating magnetic fields. Both theoretical simulation and experimental studies were performed to verify the thermotherapy effect. Experimental results showed that the temperature of the tumor center has dramatically elevated from around the room temperature to about 40oC within the first 5-10 minutes. Pathological studies demonstrate epithelial tumor cell destruction associated with the hyperthermia treatment