Articles you may be interested inCommunication: Regularizing binding energy distributions and thermodynamics of hydration: Theory and application to water modeled with classical and ab initio simulations Communication: Thermodynamics of water modeled using ab initio simulations Ab initio chemical potentials of solid and liquid solutions and the chemistry of the Earth's core We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential , rather than at constant number of electrons N e . To this end, we define the chemical potential relative to a plane ͑or ''reference electrode''͒ at a finite distance from the slab ͑the distance should reflect the particular geometry of the situation being modeled͒. To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The ''constant '' mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.

We present NECI, a state-of-the-art implementation of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm, a method based on a stochastic application of the Hamiltonian matrix on a sparse sampling of the wave function. The program utilizes a very powerful parallelization and scales efficiently to more than 24 000 central processing unit cores. In this paper, we describe the core functionalities of NECI and its recent developments. This includes the capabilities to calculate ground and excited state energies, properties via the one- and two-body reduced density matrices, as well as spectral and Green’s functions for ab initio and model systems. A number of enhancements of the bare FCIQMC algorithm are available within NECI, allowing us to use a partially deterministic formulation of the algorithm, working in a spin-adapted basis or supporting transcorrelated Hamiltonians. NECI supports the FCIDUMP file format for integrals, supplying a convenient interface to numerous quantum chemistry programs, and it is licensed under GPL-3.0.

The stability of missing-row reconstructions of ͑110͒ surfaces with respect to surface charging has been investigated using ab initio theory, taking Pt and Au as representative systems. A thermodynamic formulation is derived to compare the relative stability of charged surfaces either in constant-potential or constant-charge mode. By generalizing Koopmans' theorem to charged metallic surfaces, we obtain an expression for the surface ͑excess͒ energy as a function of charge ͑or potential͒ in terms of the neutral surface energy, work function, and the position of the image plane. A surface is shown to reconstruct in constant-charge mode if and only if it reconstructs in constant-potential mode. We next address the question of whether a positive ͑negative͒ surface charge can lift ͑induce͒ the reconstruction, as suggested in the literature. This turns out not to be the case. Instead the following consistent picture arises: at small surface charges, the effect of the charge follows the difference of the work functions; i.e., positive charge favors a surface having a smaller work function and vice versa. Larger charges, either positive or negative, tend to stabilize the reconstructed surface or, more generally, the 1ϫr reconstruction with larger r. The latter essentially results in that the 1ϫ2 reconstruction in either Pt or Au is never lifted in our study, although the 1ϫ3 surface in Au eventually becomes more stable.