Polarizable continuum model
The polarizable continuum model (PCM) is a commonly used method in computational chemistry to model solvation effects. If it were necessary to consider each solvent molecule as a separate molecule, the computational cost of modeling a solvent-mediated chemical reaction would grow prohibitively high. Modeling the solvent as a polarizable continuum, rather than individual molecules, makes ab initio computation feasible. Two types of PCMs have been popularly used: the dielectric PCM (D-PCM) in which the continuum is polarizable (see dielectrics) and the conductor-like PCM (C-PCM) in which the continuum is conductor-like similar to COSMO Solvation Model.
The molecular free energy of solvation is computed as the sum of three terms:
- Gsol = Ges + Gdr + Gcav
- Ges = electrostatic
- Gdr = dispersion-repulsion
- Gcav = cavitation
The PCM solvation model is available for calculating energies and gradients at the Hartree–Fock and density functional theory (DFT) levels in several quantum chemical computational packages such as Gaussian, GAMESS and JDFTx.
The authors of a 2002 paper observe that PCM has limitations where non-electrostatic effects dominate the solute-solvent interactions. They write in the abstract: "Since only electrostatic solute-solvent interactions are included in the PCM, our results lead to the conclusion that, for the seven molecules studied, in cyclohexane, acetone, methanol, and acetonitrile electrostatic effects are dominant while in carbon tetrachloride, benzene, and chloroform other nonelectrostatic effects are more important."
There is an integral equation formalism (IEF) version of the PCM (ref. 4 in the 2002 paper).
PCM is also used to model outer solvation layers in multi-layered solvation approach.
- Jacopo Tomasi, Benedetta Mennucci, and Roberto Cammi (2005). "Quantum Mechanical Continuum Solvation Models." Chem. Rev. 105(8): 2999-3094.
- Maurizio Cossi, Nadia Rega, Giovanni Scalmani, Vincenzo Barone (2003). "Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model." J. Comput. Chem. 24(6): 669-681.
- Hendrik Zipse (09.02.2004). "The Polarizable Continuum Model (PCM)". Retrieved January 25, 2009. Check date values in:
- B. Mennucci et al. "Polarizable Continuum Model (PCM) Calculations of Solvent Effects on Optical Rotations of Chiral Molecules." J. Phys. Chem. A 2002, 106, 6102-6113. Link to full text
- Mark S. Gordon "CLUSTER-BASED APPROACHES TO SOLVATION" Iowa State University, Ames Laboratory.