SCRF

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Solvents

Description

This keyword requests that a calculation be performed in the presence of a solvent by placing the solute in a cavity within the solvent reaction field.

The Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM) is the default SCRF method. This method creates the solute cavity via a set of overlapping spheres. It was initially devised by Tomasi and coworkers and Pascual-Ahuir and coworkers [Miertus81, Miertus82, Pascual-Ahuir94], and it has been further developed in Gaussian by the Tomasi, Barone and Mennucci groups as well as Gaussian, Inc. researchers and collaborators [Cossi96, Barone97, Cances97, Mennucci97, Mennucci97a, Barone98, Cossi98, Barone98a, Cammi99, Cossi99, Tomasi99, Cammi00, Cossi00, Cossi01, Cossi01a, Cossi02, Cossi03, Cammi09, Cammi10, Scalmani10, Lipparini10, Caricato12b]. This model corresponds to SCRF=PCM. See [Tomasi05] for a review. The model of Chipman [Chipman00] is closely related to this method [Cances01].

Gaussian also offers the SMD variation of IEFPCM of Truhlar and workers [Marenich09] via the SMD option. This is the recommended choice for computing ΔG of solvation.

Other available models are IPCM, which uses a static isodensity surface for the cavity [Foresman96], the Self-Consistent Isodensity PCM (SCIPCM) model [Foresman96], and the Onsager model [Kirkwood34, Onsager36, Wong91, Wong91a, Wong92, Wong92a], which places the solute in a spherical cavity within the solvent reaction field.

In Gaussian 16, we use 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 [Scalmani10]. This is achieved by expanding the apparent surface charge that builds up at the solute-solvent interface 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 in 1999 by Karplus and York for the conductor screening model [York99], never received the attention it deserved. We developed and generalized it within the framework of the PCM family of solvation methods in G09, and it is the default method for building the solute’s cavity and computing the reaction field.

The PCM method in Gaussian 16 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 (the latter being generated from the computed electron density with the specified model chemistry) [Improta06, Improta07]. The difference with the standard approach (based on the variational approach or linear response theory) can be illustrated with MP2. The default procedure computes the solvent effect on the SCF density and then applies MP2 perturbation, while the external iteration approach computes the solvent effect self-consistently with respect to the MP2 density. While this technique is of primary interest for studying excited state processes such fluorescence, it can also be used for ground state calculations with theoretical methods that provide gradients: e.g. post-SCF methods. Use the ExternalIteration option to specify this method.

Solvation and Excited States

There are two basic approaches available for modeling excited states in solution:

  • Computing the lowest excited states in the solvent environment. This approach adds SCRF to a normal excited state calculation such as TD or CIS. This technique employs a linear response formalism by adding the necessary terms to the excited state method equations (thereby including the solvent effects on the excited states) [Cammi00, Cossi01]. The geometry of a specific excited state can be optimized in solution with CIS or TD [Scalmani06].
  • A single excited state can be modeled via a state-specific approach. In this case, the program computes the energy in solution by making the electrostatic potential generated by the excited state density self consistent with the solvent reaction field [Improta06, Improta07], using the external iteration technique.

For excited state calculations in solution, there is a distinction between equilibrium and non-equilibrium calculations. The solvent responds in two different ways to changes in the state of the solute: it polarizes its electron distribution, which is a very rapid process, and the solvent molecules reorient themselves (e.g., by a rotation), a much slower process. An equilibrium calculation describes a situation where the solvent had time to fully respond to the solute (in both ways), e.g., a geometry optimization (a process that takes place on the same time scale as molecular motion in the solvent). A non-equilibrium calculation is appropriate for processes which are too rapid for the solvent to have time to fully respond, e.g. a vertical electronic excitation.

Equilibrium solvation is the default for CIS and TD-DFT excited state geometry optimizations. Non-equilibrium is the default for CIS and TD-DFT energies using the default PCM procedure, and equilibrium is the default for calculations using the external iteration approach (SCRF=ExternalIteration). See the examples for the method for computing non-equilibrium external iteration calculations.

For EOM-CCSD calculations in solution, the solvation approaches of Cammi and Caricato are available [Cammi09, Cammi10, Caricato12b]. The PTED method is available with the PTED option. Other methods discussed in [Caricato12b] are also available via IOp(9/116).

By default, CASSCF PCM [Cossi99] calculations correspond to equilibrium calculations with respect to the solvent reaction field/solute electronic density polarization process. Calculation of non equilibrium solute-solvent interaction involving two different electronic states (e.g. the initial and final states of a vertical transition) can be performed using the NonEq=type PCM keyword, in two separate job steps (see PCM in the Input section).

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  • Input
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  • Availability
  • Related Keywords
  • Examples
  • Additional Input
  • Solvents
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This keyword requests that a calculation be performed in the presence of a solvent by placing the solute in a cavity within the solvent reaction field.

The Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM) is the default SCRF method. This method creates the solute cavity via a set of overlapping spheres. It was initially devised by Tomasi and coworkers and Pascual-Ahuir and coworkers [Miertus81, Miertus82, Pascual-Ahuir94], and it has been further developed in Gaussian by the Tomasi, Barone and Mennucci groups as well as Gaussian, Inc. researchers and collaborators [Cossi96, Barone97, Cances97, Mennucci97, Mennucci97a, Barone98, Cossi98, Barone98a, Cammi99, Cossi99, Tomasi99, Cammi00, Cossi00, Cossi01, Cossi01a, Cossi02, Cossi03, Cammi09, Cammi10, Scalmani10, Lipparini10, Caricato12b]. This model corresponds to SCRF=PCM. See [Tomasi05] for a review. The model of Chipman [Chipman00] is closely related to this method [Cances01].

Gaussian also offers the SMD variation of IEFPCM of Truhlar and workers [Marenich09] via the SMD option. This is the recommended choice for computing ΔG of solvation.

Other available models are IPCM, which uses a static isodensity surface for the cavity [Foresman96], the Self-Consistent Isodensity PCM (SCIPCM) model [Foresman96], and the Onsager model [Kirkwood34, Onsager36, Wong91, Wong91a, Wong92, Wong92a], which places the solute in a spherical cavity within the solvent reaction field.

In Gaussian 16, we use 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 [Scalmani10]. This is achieved by expanding the apparent surface charge that builds up at the solute-solvent interface 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 in 1999 by Karplus and York for the conductor screening model [York99], never received the attention it deserved. We developed and generalized it within the framework of the PCM family of solvation methods in G09, and it is the default method for building the solute's cavity and computing the reaction field.

The PCM method in Gaussian 16 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 (the latter being generated from the computed electron density with the specified model chemistry) [Improta06, Improta07]. The difference with the standard approach (based on the variational approach or linear response theory) can be illustrated with MP2. The default procedure computes the solvent effect on the SCF density and then applies MP2 perturbation, while the external iteration approach computes the solvent effect self-consistently with respect to the MP2 density. While this technique is of primary interest for studying excited state processes such fluorescence, it can also be used for ground state calculations with theoretical methods that provide gradients: e.g. post-SCF methods. Use the ExternalIteration option to specify this method.

Solvation and Excited States

There are two basic approaches available for modeling excited states in solution:

  • Computing the lowest excited states in the solvent environment. This approach adds SCRF to a normal excited state calculation such as TD or CIS. This technique employs a linear response formalism by adding the necessary terms to the excited state method equations (thereby including the solvent effects on the excited states) [Cammi00, Cossi01]. The geometry of a specific excited state can be optimized in solution with CIS or TD [Scalmani06].
  • A single excited state can be modeled via a state-specific approach. In this case, the program computes the energy in solution by making the electrostatic potential generated by the excited state density self consistent with the solvent reaction field [Improta06, Improta07], using the external iteration technique.

For excited state calculations in solution, there is a distinction between equilibrium and non-equilibrium calculations. The solvent responds in two different ways to changes in the state of the solute: it polarizes its electron distribution, which is a very rapid process, and the solvent molecules reorient themselves (e.g., by a rotation), a much slower process. An equilibrium calculation describes a situation where the solvent had time to fully respond to the solute (in both ways), e.g., a geometry optimization (a process that takes place on the same time scale as molecular motion in the solvent). A non-equilibrium calculation is appropriate for processes which are too rapid for the solvent to have time to fully respond, e.g. a vertical electronic excitation.

Equilibrium solvation is the default for CIS and TD-DFT excited state geometry optimizations. Non-equilibrium is the default for CIS and TD-DFT energies using the default PCM procedure, and equilibrium is the default for calculations using the external iteration approach (SCRF=ExternalIteration). See the examples for the method for computing non-equilibrium external iteration calculations.

For EOM-CCSD calculations in solution, the solvation approaches of Cammi and Caricato are available [Cammi09, Cammi10, Caricato12b]. The PTED method is available with the PTED option. Other methods discussed in [Caricato12b] are also available via IOp(9/116).

By default, CASSCF PCM [Cossi99] calculations correspond to equilibrium calculations with respect to the solvent reaction field/solute electronic density polarization process. Calculation of non equilibrium solute-solvent interaction involving two different electronic states (e.g. the initial and final states of a vertical transition) can be performed using the NonEq=type PCM keyword, in two separate job steps (see PCM in the Input section).

Last updated on: 07 April 2021. [G16 Rev. C.01]