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J. L. Fattebert and F. Gygi, Int. J. Quantum Chem. 93, 139 (2003).

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O. Andreussi, I. Dabo, and N. Marzari, J. Chem. Phys. 136, 064102 (2012).

C. Dupont, O. Andreussi, and N. Marzari, J. Chem. Phys. 139, 214110 (2013).

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I. Timrov, O. Andreussi, A. Biancardi, N. Marzari and S. Baroni, J. Chem. Phys. 142, 034111 (2015).

**Environ** is a computational library aimed at introducing environment effects into atomistic first-principles simulations, in particular for applications in surface science and materials design.

A hierarchical, multiscale, strategy is at the base of the different methods implemented: while the atomistic and electronic details of the system of interest are fully preserved, the degrees of freedom of the surrounding environment (being it a liquid solution of a more complex embedding) are treated using simplified approaches. By reducing the number of degrees of freedom and by exploiting intrinsic or faster statistical averaging, the implemented methods allow the systematic un-expensive study of large systems.

**Environ** features the following approaches:

**Electrostatic effects of solvation**. By exploiting a revised version of the continuum dielectric solvation model proposed by Fattebert and Gygi (2002,2003),**Environ**allows to model the main electrostatic effects of a liquid solution on a solute, being it an isolated molecule or an incommensurate, linear or planar, system. The continuum dielectric approach, which is already a staple of quantum-chemistry calculations, is here extended to periodic and partially periodic (Andreussi 2014), possibly metallic, systems and to perform molecular dynamics simulations. Moreover, the method is easily tunable, it requires minimal parametrization, while keeping an accuracy comparable or superior to more wide-spread approaches (Andreussi 2012).**Cavitation and pressure effects**. By exploiting the definitions of quantum-volume (Cococcioni 2005) and quantum-surface (Scherlis 2006) of a system in terms of isosurfaces of its electronic density, two complex embedding effects are included in**Environ**. These features allow to model the presence of an external isotropic pressure or of a surface tension -cavitation- in the system.**Electrostatic effects due to periodic-boundary conditions**. A few of the most common approaches to remove artifacts due to periodic boundary conditions in aperiodic or partially periodic systems have been implemented in the**Environ**library. In particular, the Makov-Payne, Martyna-Tuckerman, and Point-Countercharge correction schemes have been implemented for systems in vacuum and in combination with the continuum dielectric environment (Andreussi 2014).

One common feature of the above approaches is the fact that the interface between the first-principles and the environment parts of the system are defined self-consistently in terms of the electronic density of the embedded system. An optimally smooth connection (Andreussi 2012) is used to ensure convergence of the calculations.

Each contribution can be used *per se*, to characterise individual environment effects corresponding to specific conditions: acluster under the influence of a well defined external pressure, the effect of dielectric screeneing on the electronic properties of a substrate, etc. Moreover, the **combination of different contributions** can be used to give a more complete characterization of a complex environment, such as a liquid solution. This strategy is at the core of the **Self-Consistent Continuum Solvation** (**SCCS**) approach recently proposed to model solvation effects in aqueous solutions, by including a combination of electrostatic (continuum dielectric) and non-electrostatic (dispersion-repulsion) effects by means of a parametrized combination of the different approaches of **Environ**. Results on solvation free energies of netrual (Andreussi 2012) and charged (Dupont 2014) organic molecules showed a remarkable agreement with experiments, in particular for all those compounds that do not form stable coordination compounds with water molecules and, therefore, for which a continuum description of the environment is reasonable.

**SCCS** can be used to run geometry optimizations of isolated, partially periodic and periodic systems in continuum embedding, but also to run Born-Oppeneimer molecular dynamics simulations, searches of reaction paths (through the NEB approach), and for the calculation of optical spectra (exploiting the TDDFpT approach, Timrov 2015). Extensions to Car-Parrinello molecular dynamics are in progress.