Outline of the PCM

The details of the various PCM procedures have been exhaustively reported in many papers [101,102,107], to which the reader is addressed for precise information here we shall simply sketch the main characteristics of the method. 

The partition between solute and solvent is performed by defining a cavity in the polarizable medium the solute is placed inside the cavity, where the relative dielectric constant is 1 (i. e. the value of vacuum), while outside the dielectric constant has the macroscopic value of the solvent (for example 78.39 for water at 298 K, 10.36 for 1,2-dichloroethane and so on). The shape and the size of the cavity are important parameters of the method. 

Unlike many other continuum approaches, PCM adopts cavities of realistic shape modelled on the solute atoms they are built according to GePol algorithm, [114] in which the cavity is defined as the envelope of spheres centred on solute atoms or atomic groups. Besides the atomic spheres, other spheres are added by GePol to smooth the solute-solvent boundary, approximating the so-called solvent accessible surface proposed by Connolly. 

The cavity surface is then subdivided into small domains, called surface tesserae, used to express as finite sums all the surface integrals needed to compute the solvent reaction field, as explained below. The final result is depicted in figure 3, where we show the GePol cavity for p-alanine subdivided into tesserae with average area of 0.4 and of 0.2 respectively. 

The solute-solvent interactions are accounted for by a perturbation added to the solute Hamiltonian operator  

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