Abstract The title compound has been synthesized by a new route that requires less steps and gives higher overall yields than existing procedures. The synthesis makes use of low molecular weight starting materials that are commercially available in isotopically enriched forms and thereby enables the specific labelling of carbons in the pyrimidine ring. An improved synthesis of the versatile reagent methyl propiolate is also included.
The abundant demand for deep learning compute resources has created a renaissance in low precision hardware. Going forward, it will be essential for simulation software to run on this new generation of machines without sacrificing scientific fidelity. In this paper, we examine the precision requirements of a representative kernel from quantum chemistry calculations: calculation of the single particle density matrix from a given mean field Hamiltonian (i.e. Hartree-Fock or Density Functional Theory) represented in an LCAO basis. We find that double precision affords an unnecessarily high level of precision, leading to optimization opportunities. We show how an approximation built from an error-free matrix multiplication transformation can be used to potentially accelerate this kernel on future hardware. Our results provide a road map for adapting quantum chemistry software for the next generation of High Performance Computing platforms.
Abstract This Roadmap article provides a succinct, comprehensive overview of the state of electronic structure (ES) methods and software for molecular and materials simulations. Seventeen distinct sections collect insights by 51 leading scientists in the field. Each contribution addresses the status of a particular area, as well as current challenges and anticipated future advances, with a particular eye towards software related aspects and providing key references for further reading. Foundational sections cover density functional theory and its implementation in real-world simulation frameworks, Green’s function based many-body perturbation theory, wave-function based and stochastic ES approaches, relativistic effects and semiempirical ES theory approaches. Subsequent sections cover nuclear quantum effects, real-time propagation of the ES, challenges for computational spectroscopy simulations, and exploration of complex potential energy surfaces. The final sections summarize practical aspects, including computational workflows for complex simulation tasks, the impact of current and future high-performance computing architectures, software engineering practices, education and training to maintain and broaden the community, as well as the status of and needs for ES based modeling from the vantage point of industry environments. Overall, the field of ES software and method development continues to unlock immense opportunities for future scientific discovery, based on the growing ability of computations to reveal complex phenomena, processes and properties that are determined by the make-up of matter at the atomic scale, with high precision.
Hydrovisbreaking of Athabasca bitumen has been carried out in a high shear jet reactor consisting of a high pressure stainless steel vessel equipped with a pressure nozzle. Hot H2 and bitumen were passed through the nozzle simultaneously. Due to the high shear imparted to the liquid very fine droplets were produced resulting in intimate mixing of H 2 and bitumen. Bitumen viscosity was reduced from 124000 cP to below 200 cP under a variety operating conditions. Three main variables affect the viscosity reduction: treatment temperature (420-500 ° C), reactor pressure and pressure differential across the jet nozzle. Due to the very short residence time inside the reactor, coke formation is minimal. The results are interpreted in terms of a destructuring effect caused by the high shear fields at the jet nozzle. The presence of a pyrolytic regime at the highest temperatures (> 480 ° C) favors the asphaltene conversion leading to products of lower viscosity but at the expense of some coke formation ( 1.5% wt of bitumen). It is also shown that the destructuring effect can be induced in the presence of nitrogen.
We present an approximate approach for the calculation of ionization potential (IP) and electron affinity (EA) by exploiting the complementary energy non-linearity errors for a species M and its one-electron-ionized counterpart (M+). Reasonable IPs and EAs are thus obtained by averaging the orbital energies of M and M+, even with a low-level method such as BLYP/6-31G(d). By combining the corrected IPs and EAs, we can further obtain reasonable excitation energies. The errors in uncorrected valence IPs and uncorrected virtual-orbital energies show systematic trends. These characteristics provide a convenient and computationally efficient avenue for qualitative estimation of these properties with single corrections for multiple IPs and excitation energies.
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