We extend the recently proposed heat-bath configuration interaction (HCI) method [Holmes, Tubman, Umrigar, J. Chem. Theory Comput. 2016, 12, 3674], by introducing a semistochastic algorithm for performing multireference Epstein-Nesbet perturbation theory, in order to completely eliminate the severe memory bottleneck of the original method. The proposed algorithm has several attractive features. First, there is no sign problem that plagues several quantum Monte Carlo methods. Second, instead of using Metropolis-Hastings sampling, we use the Alias method to directly sample determinants from the reference wave function, thus avoiding correlations between consecutive samples. Third, in addition to removing the memory bottleneck, semistochastic HCI (SHCI) is faster than the deterministic variant for many systems if a stochastic error of 0.1 mHa is acceptable. Fourth, within the SHCI algorithm one can trade memory for a modest increase in computer time. Fifth, the perturbative calculation is embarrassingly parallel. The SHCI algorithm extends the range of applicability of the original algorithm, allowing us to calculate the correlation energy of very large active spaces. We demonstrate this by performing calculations on several first row dimers including F with an active space of (14e, 108o), Mn-Salen cluster with an active space of (28e, 22o), and Cr dimer with up to a quadruple-ζ basis set with an active space of (12e, 190o). For these systems we were able to obtain better than 1 mHa accuracy with a wall time of merely 55 s, 37 s, and 56 min on 1, 1, and 4 nodes, respectively.

The growing hydrogen-economy requires accelerating the hydrogen evolution reaction. The water dissociation step (Volmer step) has been proposed as a main kinetic limitation, but the mechanisms at play in the electrochemical double-layer are poorly understood. This is due to the ambivalent role of water: it acts both as a reactant and as a solvent. Here we propose to confine water inside an organic liquid matrix in order to isolate the sole role of water as a reactant. We observe the formation of aqueous-rich nanodomains, which size can be tuned by changing the supporting electrolyte, and found that the reactivity of the system significantly varies with its nanostructure. Depending on the conditions, it is dominated by either the strength of short-range cation-water interactions or the formation of long chains of water molecules. This understanding paves the way toward the development of more efficient and selective electrocatalysts for the water, CO 2 , O 2 or N 2 reduction. File list (3) download file view on ChemRxiv Tuning the water reduction through controlled nanoconfine... (4.83 MiB) download file view on ChemRxiv Tuning the water reduction through controlled nanoconfine... (2.81 MiB) download file view on ChemRxiv data_MD_Dubouis_et_al_2020_Nat_Catal.zip (24.94 MiB)