Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Communication: Analytic gradients in the random-phase approximation

The relationship between the random-phase-approximation (RPA) correlation energy and the continuous algebraic Riccati equation is examined and the importance of a stabilizing solution is emphasized. The criterion to distinguish this from non-stabilizing solutions can be used to ensure that physical, smooth potential energy surfaces are obtained. An implementation of analytic RPA molecular gradients is presented using the Lagrangian technique. Illustrative calculations indicate that RPA with Hartree-Fock reference orbitals delivers an accuracy similar to that of second-order M\uf8ller\u2013 Plesset perturbation theor