IPCM,

The original PCM method uses a cavity made of spherical regions around each atom. The isodensity PCM model (IPCM) uses a cavity that is defined by an isosurface of the electron density. This is defined iteratively by running SCF calculations with the cavity until a convergence is reached. The self-consistent isodensity PCM model (SCI-PCM) is similar to IPCM in theory, but different in implementation. SCI-PCM calculations embed the cavity calculation in the SCF procedure to account for coupling between the two parts of the calculation. 

The Isodensity PCM (IPCM) model defines the cavity as an isodensity surface of the molecule. This isodensity is determined by an iterative process in which an SCF cycle is performed and converged using the current isodensity cavity. The resultant wavefunction is then used to compute an updated isodensity surface, and the cycle is repeated until the cavity shape no longer changes upon completion of the SCF. 

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cyclohexane (e=2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (flfl=3.65) and using the IPCM model at the B3LYP level. The 6-31+G(d) basis set was used for all jobs. We also ran gas phase calculations for both conformations at the same model chemistries, and an IPCM calculation for the trans conformation (SCRF=Dipole calculations are not necessary for the trans conformation since it has no dipole moment). 

Medium gas phase HF MP2 B3LYP gcyclohexane Onsager IPCM HF MP2 B3LYP ... 

As the plot of AE indicates, the energy difference between the two forms decreases in more polar solvents, and becomes nearly zero in acetonitrile. The left plot illustrates the fact that the IPCM model (at the B3LYP/6-31+G(d) level of theory) does a much better job of reproducing the observed solvent effect than the two Onsager SCRF models. In contrast, the Onsager model at the MP2 level treats the solvated systems more accurately than it does the gas phase system, leading to a poorer value for the solvent effect. ... 

Solution The following table lists the energy differences that we computed as well as the original researchers HF/6-31+G(d) Onsager and B3LYP/6-31+G(d) SCRF=IPCM results ... 

Both the MP2 Onsager calculation and the IPCM calculaton are in good agreement with experiment. The SCI-PCM and Hartree-Fock Onsager SCRF calculations perform significantly less well for this problem. ... 

J. B. Foresman, T. A. Keith, K. B. Wiberg, J. Snoonian and M. J. Frisch, Solvent Effects. 5. The Influence of Cavity Shape, Truncation of Electrostatics, and Electron Correlation on Ab Initio Reaction Field Calculations, J. Phys. Chem., submitted (1996). [Discusses the IPCM SCRF model.]... 

Over the years, many workers have addressed the problem of choice of cavity and the reaction field. Tomasi s polarized continuum model (PCM) defines the cavity as a series of interlocking spheres. The isodensity PCM (IPCM) defines the cavity as an isodensity surface of the molecule. This isodensity surface is determined iteratively. The self-consistent isodensity polarized continuum model (SQ-PCM) gives a further refinement in that it allows for a full coupling between the cavity shape and the electron density. 

Universite de Strasbourg IPCMS-CNRS 23 Rue du Loess 67034 Strasbourg France... 

Fig. 7. Energy profile for water exchange around the beryllium dication (B3LYP/6-311+G", italic B3LYP(IPCM)/6-311+G") (156).

In the IPCM calculations, the molecule is contained inside a cavity within the polarizable continuum, the size of which is determined by a suitable computed isodensity surface. The size of this cavity corresponds to the molecular volume allowing a simple, yet effective evaluation of the molecular activation volume, which is not based on semi-empirical models, but also does not allow a direct comparison with experimental data as the second solvation sphere is almost completely absent. The volume difference between the precursor complex Be(H20)4(H20)]2+ and the transition structure [Be(H20)5]2+, viz., —4.5A3, represents the activation volume of the reaction. This value can be compared with the value of —6.1 A3 calculated for the corresponding water exchange reaction around Li+, for which we concluded the operation of a limiting associative mechanism. In the present case, both the nature of [Be(H20)5]2+ and the activation volume clearly indicate the operation of an associative interchange mechanism (156). 

The weakest point of our approach is the treatment of the bulk solvent. The energies derived from an implicit solvent model like IPCM are mainly based on energy calculations on gas-phase structures and effects of explicit solvent molecules are not included. 

Including solvent effects employing B3LYP(IPCM)/6-311+G 7/ B3LYP/6-311+G energy calculations, significantly increases the reaction energies, independent of whether water or acetonitrile is selected as solvent (see Table V) (93). 

B3LYP(IPCM)/6-311+G 7/B3LYP/ 6-311+G Gas phase IPCM acetonitrile IPCM water... 

While the MP2(full)/6-311+G 7/B3LYP/6-311+G energies show typical discrepancies, the application of the IPCM- and CPCM-solvent models dramatically lowers the energy for the intermediate and the transition states. The barriers of 2.8 and 1.8kcalmol 1 corroborate the experimental findings for an efficient formation of a water coordinated Li+ complex. 

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