A Localized-Orbital Energy Evaluation for Auxiliary-Field Quantum Monte Carlo.

Phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) has recently emerged as a promising method for the production of benchmark-level simulations of medium- to large-sized molecules because of its accuracy and favorable polynomial scaling with system size. Unfortunately, the memory footprints of standard energy evaluation algorithms are nontrivial, which can significantly impact timings on graphical processing units (GPUs) where memory is limited. Previous attempts to reduce scaling by taking advantage of the low-rank structure of the Coulombic integrals have been successful but exhibit high prefactors, making their utility limited to very large systems. Here we present a complementary cubic-scaling route to reduce memory and computational scaling based on the low rank of the Coulombic interactions between localized orbitals, focusing on the application to ph-AFQMC. We show that the error due to this approximation, which we term localized-orbital AFQMC (LO-AFQMC), is systematic and controllable via a single variable and that the method is computationally favorable even for small systems. We present results demonstrating robust retention of accuracy versus both experiment and full ph-AFQMC for a variety of test cases chosen for their potential difficulty for localized-orbital-based methods, including the singlet-triplet gaps of the polyacenes benzene through pentacene, the heats of formation for a set of Platonic hydrocarbon cages, and the total energy of ferrocene, Fe(Cp)2. Finally, we reproduce our previous result for the gas-phase ionization energy of Ni(Cp)2, agreeing with full ph-AFQMC to within statistical error while using less than 1/15th of the computer time.

Calculation of Metallocene Ionization Potentials via Auxiliary Field Quantum Monte Carlo: Toward Benchmark Quantum Chemistry for Transition Metals.

The accurate ab initio prediction of ionization energies is essential to understanding the electrochemistry of transition metal complexes in both materials science and biological applications. However, such predictions have been complicated by the scarcity of gas phase experimental data, the relatively large size of the relevant molecules, and the presence of strong electron correlation effects. In this work, we apply all-electron phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) utilizing multideterminant trial wave functions to six metallocene complexes to compare the computed adiabatic and vertical ionization energies with experimental results. We find that ph-AFQMC yields mean absolute errors (MAEs) of 1.69 ± 1.02 kcal/mol for the adiabatic energies and 2.85 ± 1.13 kcal/mol for the vertical energies. We also carry out density functional theory (DFT) calculations using a variety of functionals, which yields MAEs of 3.62-6.98 kcal/mol and 3.31-9.88 kcal/mol, as well as one variant of localized coupled cluster calculations (DLPNO-CCSD(T0) with moderate PNO cutoffs), which has MAEs of 4.96 and 6.08 kcal/mol, respectively. We also test the reliability of DLPNO-CCSD(T0) and DFT on acetylacetonate (acac) complexes for adiabatic energies measured in the same manner experimentally, and we find higher MAEs, ranging from 4.56 to 10.99 kcal/mol (with a different ordering) for DFT and 6.97 kcal/mol for DLPNO-CCSD(T0). Finally, by utilizing experimental solvation energies, we show that accurate reduction potentials in solution for the metallocene series can be obtained from the AFQMC gas phase results.

Chemically Tuned, Reversible Fluorogenic Electrophile for Live Cell Nanoscopy.

We report a chemically tuned fluorogenic electrophile designed to conduct live-cell super-resolution imaging by exploiting its stochastic reversible alkylation reaction with cellular nucleophiles. Consisting of a lipophilic BODIPY fluorophore tethered to an electrophilic cyanoacrylate warhead, the new probe cyanoAcroB remains nonemissive due to internal conversion along the cyanoacrylate moiety. Intermittent fluorescence occurs following thiolate Michael addition to the probe, followed by retro-Michael reaction, tuned by the cyano moiety in the acrylate warhead and BODIPY decoration. This design enables long-term super-resolved imaging of live cells by preventing fluorescent product accumulation and background increase, while preserving the pool of the probe. We demonstrate the imaging capabilities of cyanoAcroB via two methods: (i) single-molecule localization microscopy imaging with nanometer accuracy by stochastic chemical activation and (ii) super-resolution radial fluctuation. The latter tolerates higher probe concentrations and low imaging powers, as it exploits the stochastic adduct dissociation. Super-resolved imaging with cyanoAcroB reveals that electrophile alkylation is prevalent in mitochondria and endoplasmic reticulum. The 2D dynamics of these organelles within a single cell are unraveled with tens of nanometers spatial and sub-second temporal resolution through continuous imaging of cyanoAcroB extending for tens of minutes. Our work underscores the opportunities that reversible fluorogenic probes with bioinspired warheads bring toward illuminating chemical reactions with super-resolved features in live cells.