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

Rice research and development in West Africa: problems and perspectives.

The place of the rice economy in West Africa and its development since the 1960s is considered on and its disappointing performance is reviewed within the context of the area's overall food problems. Rice research and development in West Africa is briefly described and in relation to major agro-ecological zones. Constraints and potentials are discussed and the question as to why returns from rice research are so insignificant is posed. Finally, the perspectives of improved research allocation and productivity in regional rice research are discussed. (Abstract retrieved from CAB Abstracts by CABI’s permission

Ownership structure impact on exchange rate exposure under the RMB regime reform

This thesis investigates effects of exchange rate exposure upon stock returns of a large sample of Chinese firms when they are divided into SOEs and non-SOEs. I obtain the exposure estimates first and examine their determinants. By extending the current literature, I introduce the ownership structure into the framework and I compare different ownership impacts on the exposure before and after the Reform. My study shows that a SOE or non-SOE firm makes no difference in facing the exchange rate exposure before the Reform. If a firm is a SOE, the firm faces smaller exposure after the Reform. I further explore the asymmetric exposure from currency depreciation and appreciation in relation to ownership structure. I find that non-SOEs face higher exposure from appreciation than from depreciation before the Reform. But both SOEs and non-SOEs face lower exposure from appreciation than from depreciation after the Reform

Interdisciplinary thinking in agricultural and life sciences higher education

Interdisciplinary thinking as a skill appears to be of value to higher education students and those in employment. This idea is explored with reference to the agricultural and life sciences. The need for further understanding of the development of interdisciplinary thinking is acknowledged. This is closely related to the requirement for well-founded curriculum and course design. This publication presents a brief introduction to a systematic review of scientific research into teaching and learning in interdisciplinary higher education. While tentative, the understanding arising from the review findings is considered to be of potential value to educational practice. A selection of the review findings is presented by way of illustration. The selection is believed to be of relevance to the agricultural and life sciences. The review findings presented here take the form of interdisciplinary thinking sub skills and enabling condition