Gaussian 09 Scrf
Julieann Rohde
Gaussian 09 SCRF: A Solvation Model for Computational Chemistry
Gaussian 09 is a software package for performing electronic structure calculations on molecules and solids. One of the features of Gaussian 09 is the ability to perform calculations in the presence of a solvent by using the self-consistent reaction field (SCRF) method. The SCRF method approximates the effect of the solvent on the solute by placing the solute in a cavity within the solvent and applying a dielectric continuum model to account for the polarization of the solvent molecules by the solute charge distribution.
There are different variants of the SCRF method available in Gaussian 09, each with its own advantages and limitations. The default SCRF method is the polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM). This method creates the solute cavity by using a set of overlapping spheres centered on the solute atoms, and solves an integral equation to obtain the apparent surface charge (ASC) that builds up at the solute-solvent interface. The ASC is then used to compute the reaction field potential and energy of the solute in solution.
The PCM method in Gaussian 09 uses a continuous surface charge formalism that ensures continuity, smoothness and robustness of the reaction field, which also has continuous derivatives with respect to atomic positions and external perturbing fields . This is achieved by expanding the ASC in terms of spherical Gaussian functions located at each surface element in which the cavity surface is discretized. Discontinuities in the surface derivatives are removed by effectively smoothing the regions where the spheres intersect. This formalism, initially proposed by Karplus and York for the conductor screening model , was developed and generalized within the framework of the PCM family of solvation methods by Scalmani et al. .
The PCM method also includes an external iteration procedure whereby the program computes the energy in solution by making the solvent reaction field self-consistent with the solute electrostatic potential . This technique is especially useful for studying excited state processes such as fluorescence, but it can also be used for ground state calculations with post-SCF methods that provide gradients, such as MP2 or DFT.
Other SCRF methods available in Gaussian 09 are IPCM, which uses a static isodensity surface for the cavity , SCIPCM, which uses a self-consistent isodensity surface , and Onsager, which places the solute in a spherical cavity . Gaussian 09 also offers the SMD variation of IEFPCM of Truhlar and workers via the SMD option. This is the recommended choice for computing free energies of solvation.
To perform an SCRF calculation in Gaussian 09, one needs to specify the SCRF keyword with an appropriate option for the solvation model and additional input for defining the solvent properties and parameters. For example, to perform a B3LYP/6-311+G(2d,p) calculation on water in chloroform using PCM with external iteration, one would use:
# B3LYP/6-311+G(2d,p) SCRF=(PCM,Solvent=Chloroform,ExternalIteration)The solvent can be specified by name if it is one of the predefined solvents in Gaussian 09, or by its dielectric constant if it is not. For example, to use acetone as a solvent, one would use:
# B3LYP/6-311+G(2d,p) SCRF=(PCM,Solvent=Acetone)or# B3LYP/6-311+G(2d,p) SCRF=(PCM,Solvent=(Dielectric=20.7))More details on how to use SCRF methods in Gaussian 09 can be found in the official documentation.
References
- Scalmani G., Frisch M.J., Mennucci B., Tomasi J., Cammi R., Barone V., J. Chem. Phys., 132 (2010), 114110.
- York D.M., Karplus M., J. Phys e8e8a447ac