11.2.4 PCM Job Control

Theory
       Specifies the which polarizable continuum model will be used.
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       CPCM
OPTIONS:
       CPCM Conductor-like PCM with $f_{\epsilon }=(\epsilon -1)/\epsilon$. COSMO Original conductor-like screening model with $f_{\epsilon }=(\epsilon -1)/(\epsilon +1/2)$. IEFPCM IEF-PCM with an asymmetric $𝐊$ matrix. SSVPE SS(V)PE model, equivalent to IEF-PCM with a symmetric $𝐊$ matrix.
RECOMMENDATION:
       The IEF-PCM/SS(V)PE model is more sophisticated model than either C-PCM or COSMO, and probably more appropriate for low-dielectric solvents, but it is also more computationally demanding. In high-dielectric solvents there is little difference between these models. Note that the keyword COSMO in this context simply affects the dielectric screening factor $f_{\epsilon }$; to obtain the outlying charge correction suggested by Klamt, ⁶²⁵ Klamt A., Jonas V.
J. Chem. Phys.
(1996), 105, pp. 9972. Link ^{, 62} Baldridge K., Klamt A.
J. Chem. Phys.
(1997), 106, pp. 6622. Link one should use SOLVENT_METHOD = COSMO rather than SOLVENT_METHOD = PCM; see Section 11.2.8.

Method
       Specifies which surface discretization method will be used.
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       SwiG
OPTIONS:
       SwiG Switching/Gaussian method ISwiG “Improved” Switching/Gaussian method with an alternative switching function Spherical Use a single, fixed sphere for the cavity surface. Fixed Use discretization point charges instead of smooth Gaussians.
RECOMMENDATION:
       Use of SwiG is recommended only because it is slightly more efficient than the switching function of ISwiG. On the other hand, ISwiG offers some conceptually more appealing features and may be superior in certain cases. Consult Refs.  683 Lange A. W., Herbert J. M.
J. Chem. Phys.
(2010), 133, pp. 244111. Link , 685 Lange A. W., Herbert J. M.
Chem. Phys. Lett.
(2011), 509, pp. 77. Link for a discussion of these differences. The Fixed option uses the Variable Tesserae Number (VTN) algorithm of Li and Jensen, ⁷³⁶ Li H., Jensen J. H.
J. Comput. Chem.
(2004), 25, pp. 1449. Link with Lebedev grid points. VTN uses point charges with no switching function or Gaussian blurring, and is therefore subject to discontinuities that may cause errors in analytic gradients and lead to slow convergence in geometry optimizations. ⁶⁸² Lange A. W., Herbert J. M.
J. Phys. Chem. Lett.
(2010), 1, pp. 556. Link Its use is not recommended except to make contact with other calculations in the literature.

Solver
       Specifies the algorithm used to solve the PCM equations.
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       INVERSION
OPTIONS:
       INVERSION Direct matrix inversion CG Iterative conjugate gradient
RECOMMENDATION:
       Matrix inversion is faster for small solutes because it needs to be performed only once in a single-point calculation. However, the CG solver (which must be applied at each SCF iteration) is recommended if the number of discretization points approaches or exceeds $\sim$10,000. Linear-scaling options for large-scale MM/PCM and QM/MM/PCM calculations are discussed in Section 11.2.5.

SwitchThresh
       Threshold for discarding grid points on the cavity surface.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       8
OPTIONS:
       $n$ Discard grid points when the switching function is less than ${10}^{-n}$.
RECOMMENDATION:
       Use the default, which is found to avoid discontinuities within machine precision. Increasing $n$ reduces the cost of PCM calculations but can introduce discontinuities in the potential energy surface. If the PCM part of the calculation becomes too expensive it is better to reduce the number of discretization points (HeavyPoints and HPoints, described below), or to use Solver = CG, than it is to adjust SwitchThresh.

Radii
       Specifies which set of atomic van der Waals radii will be used to define the solute cavity.
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       Bondi
OPTIONS:
       Bondi Use the (extended) set of Bondi radii. FF Use Lennard-Jones radii from a molecular mechanics force field. UFF Use radii from the Universal Force Field. Read Read the atomic radii from a $van_der_waals input section.
RECOMMENDATION:
       Bondi radii are widely used. The FF option requires the user to specify an MM force field using the FORCE_FIELD $rem variable, and also to define the atom types in the $molecule section (see Section 11.3). This is not required for UFF radii.

vdwScale
       Scaling factor for the atomic van der Waals radii used to define the solute cavity.
INPUT SECTION: $pcm
TYPE:
       FLOAT
DEFAULT:
       1.2
OPTIONS:
       $\alpha$ Use a scaling factor of $\alpha >0$.
RECOMMENDATION:
       The default value is widely used in PCM calculations, although a value of 1.0 might be appropriate if using a solvent-accessible surface.

SASradius
       Form a “solvent accessible” surface with the given solvent probe radius.
INPUT SECTION: $pcm
TYPE:
       FLOAT
DEFAULT:
       0.0
OPTIONS:
       $r$ Use a solvent probe radius of $r$, in Å.
RECOMMENDATION:
       The solvent probe radius is added to the scaled van der Waals radii of the solute atoms. A common solvent probe radius for water is 1.4 Å, but smaller values (0.2–0.5 Å) have also been used historically. ⁵⁰⁵ Herbert J. M.
Wiley Interdiscip. Rev.: Comput. Mol. Sci.
(2021), 11, pp. e1519. Link

SurfaceType
       Selects the solute cavity surface construction.
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       VDW_SAS
OPTIONS:
       VDW_SAS van der Waals or solvent-accessible surface SES solvent-excluded surface
RECOMMENDATION:
       The vdW surface and the SAS are each comprised simply of atomic spheres and thus share a common option; the only difference is the specification of a solvent probe radius, SASradius. For a true vdW surface, the probe radius should be zero (which is the default), whereas for the SAS the atomic radii are traditionally not scaled, hence vdwScale should be set to 1.0 (which is not the default). For the SES, only SwiG discretization is available, but this can be used with any set of (scaled or unscaled) atomic radii, or with radii that are augmented by SASradius.

HeavyPoints
       The number of Lebedev grid points to be placed non-hydrogen atoms in the QM system.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       194
OPTIONS:
       Acceptable values are listed above.
RECOMMENDATION:
       Use the default for geometry optimizations. For absolute solvation energies, the user may want to examine convergence with respect to $N$.

HPoints
       The number of Lebedev grid points to be placed on H atoms in the QM system.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       110
OPTIONS:
       Acceptable values are listed above.
RECOMMENDATION:
       Use the default for geometry optimizations. For absolute solvation energies, the user may want to examine convergence with respect to $N$.

MMHeavyPoints
       The number of Lebedev grid points to be placed on non-hydrogen atoms in the MM subsystem.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       110
OPTIONS:
       Acceptable values are listed above.
RECOMMENDATION:
       Use the default for geometry optimizations. For absolute solvation energies, the user may want to examine convergence with respect to $N$. This option applies only to MM/PCM or QM/MM/PCM calculations.

MMHPoints
       The number of Lebedev grid points to be placed on H atoms in the MM subsystem.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       50
OPTIONS:
       Acceptable values are listed above.
RECOMMENDATION:
       Use the default for geometry optimizations. For absolute solvation energies, the user may want to examine convergence with respect to $N$. This option applies only to MM/PCM or QM/MM/PCM calculations.

PrintLevel
       Controls the printing level during PCM calculations.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Prints PCM energy and basic surface grid information. Minimal additional printing. 1 Level 0 plus PCM solute-solvent interaction energy components and Gauss’ Law error. 2 Level 1 plus surface grid switching parameters and a .PQR file for visualization of the cavity surface apparent surface charges. 3 Level 2 plus a .PQR file for visualization of the electrostatic potential at the surface grid created by the converged solute. 4 Level 3 plus additional surface grid information, electrostatic potential and apparent surface charges on each SCF cycle. 5 Level 4 plus extensive debugging information.
RECOMMENDATION:
       Use the default unless further information is desired.

CavityRadius
       Specifies the solute cavity radius.
INPUT SECTION: $pcm
TYPE:
       FLOAT
DEFAULT:
       None
OPTIONS:
       $R$ Use a radius of $R$, in Ångstroms.
RECOMMENDATION:
       None.

CavityCenter
       Specifies the center of the spherical solute cavity.
INPUT SECTION: $pcm
TYPE:
       FLOAT
DEFAULT:
       0.0 0.0 0.0
OPTIONS:
       $x$ $y$ $z$ Coordinates of the cavity center, in Ångstroms.
RECOMMENDATION:
       The format is CavityCenter followed by three floating-point values, delineated by spaces. Uses the same coordinate system as the $molecule section.

Dielectric
       Specifies the static dielectric constant for the PCM solvent.
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       78.39
OPTIONS:
       $\epsilon$ Use a dielectric constant $\epsilon >0$.
RECOMMENDATION:
       The static (i.e., zero-frequency) dielectric constant is what is usually called “the” dielectric constant. The default corresponds to water at 25${}^{\circ }$C.

OpticalDielectric
       Specifies the optical dielectric constant for the PCM solvent.
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       1.78
OPTIONS:
       ${\epsilon }_{\mathrm{\infty }}$ Use an optical dielectric constant ${\epsilon }_{\mathrm{\infty }}>0$.
RECOMMENDATION:
       The default corresponds to water at 25${}^{\circ }$C. Note that ${\epsilon }_{\mathrm{\infty }}=n^2$, where $n$ is the solvent’s index of refraction.

SolventName
       Specify both $\epsilon$ and ${\epsilon }_{\mathrm{\infty }}$ for PCM calculations by naming the solvent.
INPUT SECTION: $solvent
TYPE:
       STRING
DEFAULT:
       NONE
OPTIONS:
       name Use values $\epsilon$ and ${\epsilon }_{\mathrm{\infty }}$ corresponding to solvent name in Table 11.4.
RECOMMENDATION:
       The use of SolventName is incompatible with explicit specification of either Dielectric or OpticalDielectric.

NonEls
       Specifies what type of nonelectrostatic contributions to include.
INPUT SECTION: $solvent
TYPE:
       STRING
DEFAULT:
       None
OPTIONS:
       Cav Cavitation energy Buck Buckingham dispersion and repulsion energy from atomic number LJ Lennard-Jones dispersion and repulsion energy from force field BuckCav Buck + Cav LJCav LJ + Cav
RECOMMENDATION:
       A very limited set of parameters for the Buckingham potential is available at present.

NSolventAtoms
       The number of different types of atoms.
INPUT SECTION: $solvent
TYPE:
       INTEGER
DEFAULT:
       None
OPTIONS:
       $N$ Specifies that there are $N$ different types of atoms.
RECOMMENDATION:
       This keyword is necessary when NonEls = Buck, LJ, BuckCav, or LJCav. Methanol (${\mathrm{CH}}_3\mathrm{OH}$), for example, has three types of atoms (C, H, and O).

SolventAtom
       Specifies a unique solvent atom.
INPUT SECTION: $solvent
TYPE:
       Various
DEFAULT:
       None.
OPTIONS:
       Input (TYPE) Description Number1 (INTEGER): The atomic number of the atom Number2 (INTEGER): How many of this atom are in a solvent molecule Number3 (INTEGER): Force field atom type SASradius (FLOAT): Probe radius (in Å) for defining the solvent accessible surface
RECOMMENDATION:
       If not using LJ or LJCav, then Number3 should be set to 0. The SolventAtom keyword is necessary when NonEls = Buck, LJ, BuckCav, or LJCav

Temperature
       Specifies the solvent temperature.
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       300.0
OPTIONS:
       $T$ Use a temperature of $T$, in Kelvin.
RECOMMENDATION:
       Used only for the cavitation energy.

Pressure
       Specifies the solvent pressure.
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       1.0
OPTIONS:
       $P$ Use a pressure of $P$, in bar.
RECOMMENDATION:
       Used only for the cavitation energy.

SolventRho
       Specifies the solvent number density
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       Determined for water, based on temperature.
OPTIONS:
       $\rho$ Use a density of $\rho$, in molecules/Å${}^3$.
RECOMMENDATION:
       Used only for the cavitation energy.

SolventRadius
       The radius of a solvent molecule of the PCM solvent.
INPUT SECTION: $solvent
TYPE:
       FLOAT
DEFAULT:
       None
OPTIONS:
       $r$ Use a radius of $r$, in Å.
RECOMMENDATION:
       Used only for the cavitation energy.

ChargeSeparation
       Partition fast and slow charges in solvent equilibrium state
INPUT SECTION: $pcm
TYPE:
       STRING
DEFAULT:
       No default.
OPTIONS:
       Marcus Do slow-fast charge separation with the Marcus partition. Pekar Do slow-fast charge separation with the Pekar partition.
RECOMMENDATION:
       Charge separation is used in conjunction with the StateSpecific keyword in $pcm.

StateSpecific
       Specifies which the state-specific PCM will be used.
INPUT SECTION: $pcm
TYPE:
       Various
DEFAULT:
       No default.
OPTIONS:
       Perturb Perform ptSS and ptLR for vertical excitations. External Perform self-consistent EI-SS-PCM to the excited state (for emission). Internal Perform self-consistent II-SS-PCM to the excited state (for emission or excitation).
RECOMMENDATION:
       Use for vertical excition energies ptSS-PCM or in very polar cases the nonequilibrium II-SS-PCM, and equilibrium EI-SS-PCM for emission energies of long-lived excited states.

TdNonEq
       Specify the self-consistent SS-PCM/TDDFT method to calculate the solvent effects on vertical absorption and emission in solution based on the constrained equilibrium principle for nonequilibrium solvation.
INPUT SECTION: $pcm
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       1 Calculate nonequilibrium excited-state free energy in absorption based on the equilibrium ground-state reaction field via an RPA calculation at the ground-state geometry. 2 Calculate nonequilibrium excited-state free energy in absorption based on the nonequilibrium excited-state reaction field via an RPA calculation at the ground-state geometry. 3 Calculate equilibrium excited-state free energy in emission based on the equilibrium excited-state reaction field via an RPA calculation at the excited-state geometry. 4 Calculate nonequilibrium ground-state free energy in emission based on the nonequilibrium ground-state reaction field via a pure single-point energy calculation at the excited-state geometry.
RECOMMENDATION:
       Option 2 and 3 need to be iterated until the excited-state reaction field converges. Option 1 is the first iteration of option 2. After the equilibrium excited state reaction field of option 3 converges, option 4 is executed.