SCRF Keyword
Last Update: 06/04/2001

DESCRIPTION

This keyword requests that a calculation be performed in the presence of a solvent, using an Onsager model [150,151,152,153,275-276], the Polarized Continuum (overlapping spheres) model (PCM) of Tomasi and coworkers [154, 155,343, 347, 346, 345, 350], a (static) isodensity surface polarized continuum model (IPCM) [156] or a Self-Consistent Isodensity PCM (SCI-PCM) model [209].

REQUIRED INPUT: ONSAGER MODEL

For the Onsager model (SCRF=Dipole), the solute radius in Angstroms and the dielectric constant of the solvent are read as two free-format real numbers on one line from the input stream. A suitable solute radius is computed by a gas-phase molecular volume calculation (in a separate job step); see the discussion of the Volume keyword below.

REQUIRED INPUT: IPCM AND SCI-PCM MODELS

For the IPCM and SCI-PCM models, the input consists of a line specifying the dielectric constant of the solvent and an optional isodensity value (the default for the latter is 0.0004).

REQUIRED AND OPTIONAL INPUT: PCM MODELS

Keywords and options specifying details for a PCM calculation (SCRF=PCM, DPCM, CPCM or IEFPCM) may be specified in an additional blank-line terminated input section provided that the Read option is also specified. Keywords within this section follow general Gaussian input rules. The available keywords are listed at the end of the manual section for the SCRF keyword.

OPTION FOR SPECIFYING THE SOLVENT

Solvent=item Selects the solvent in which the calculation is to be performed. Note that the solvent may also be specified in the input stream in various ways for the different SCRF methods. If unspecified, the solvent defaults to water. Item may be either a solvent name or solvent ID number chosen from the following list:

METHOD SELECTION OPTIONS

Dipole
Perform an Onsager model reaction field calculation. This is the default.

PCM
Perform a PCM model reaction field calculation using the polarizable dielectric model [155, 154, 343 ]. DPCM, Tomasi and Pisa are synonyms for this option.

CPCM
Perform a PCM calculation using the polarizable conductor calculation model [350]. Cosmo is a synonym for this option.

IEFPCM
Perform a PCM calculation using the integral equation formalism model [345, 347,352].

IPCM
Perform an IPCM model reaction field calculation. Isodensity is a synonym for IPCM.

SCIPCM
Perform an SCI-PCM model reaction field calculation: perform an SCRF calculation using a cavity determined self-consistently from an isodensity surface. This is the default for single point energy calculations and optimizations.

DIPOLE MODEL OPTIONS

A0=val
Sets the value for the solute radius in the route section (rather than reading it from the input stream). If this option is included, then Solvent or Dielectric must also be included.

Dielectric=val
Sets the value for the dielectric constant of the solvent. This option overrides Solvent if both are specified.

PCM MODELS OPTION

Read
Indicates that a separate section of keywords and options providing calculation parameters should be read from the input stream (as described above).

IPCM MODEL OPTIONS

GradVne
Use Vne basins for the numerical integration.

GradRho
Use density basins for the numerical integration. The job may fail if non-nuclear attractors are present.

SCI-PCM MODEL OPTIONS

UseDensity
Force the use of the density matrix in evaluating the density.

UseMOs
Force the use of MOs in evaluating the density.

GasCavity
Use the gas phase isodensity surface to define the cavity rather than solving for the surface self-consistently. This is mainly a debugging option.

NoScale
Turn off scaling (designed to account for charge outside the cavity) for the SCI-PCM calculation. Scaling is performed by default. NoScale reproduces the behavior of SCI-PCM in Gaussian 94.

NUMERICAL SCRF OPTIONS

Numer
Force numerical SCRF rather than analytic. This keyword is required for multiple orders beyond Dipole

Dipole
The options Dipole, Quadrupole, Octopole, and Hexadecapole specify the order of multipole to use in the SCRF calculation. All but Dipole require that the Numer option be specified as well.

Checkpoint
Begin the numerical SCRF with a previously computed reaction field from the checkpoint file. This is synonymous with Field=EChk.

Cards
Begin the numerical SCRF with a previously computed reaction field read from the input stream, immediately after the line specifying the dielectric constant and radius (three free-format reals).

AVAILABILITY AND RESTRICTIONS

The Onsager model is available for HF, DFT, MP2, MP3, MP4(SDQ), QCISD, CCD, CID, and CISD energies, and for HF and DFT optimizations and frequency calculations.

The PCM and IPCM models are available for HF, DFT, MP2, MP3, MP4(SDQ), QCISD, CCD, CID, and CISD energies only.

The SCI-PCM model is available for HF and DFT energies and optimizations and numerical frequencies.

The Opt Freq keyword combination may not be used in SCRF calculations.

SCRF=PCM and SCRF=IPCM jobs can be restarted from the checkpoint file by using the Restart keyword in the job¹s route section. SCRF=SCIPCM calculations which fail during the SCF iterations should be restarted via the SCF=Restart keyword.

RELATED KEYWORDS

Volume, SCF

EXAMPLES

The energy computed by an Onsager SCRF calculation appears in the output file as follows:

Total energy (include solvent energy) =   -74.95061789532

Energy output from the other SCRF models appears in the normal way within the output file, followed by additional information about the calculation. For example, here is the section of the output file containing the predicted energy from a PCM calculation:

IN VACUO Dipole moment (Debye):
   X=  0.0000  Y=  0.0000  Z= -2.0683  Tot=  2.0683
 IN SOLUTION Dipole moment (Debye):
   X=  0.0000  Y=  0.0000  Z= -2.1876  Tot=  2.1876
 SCF Done:  E(RHF) =  -100.029187240     A.U. after    5 cycles
             Convg  =    0.4249D-05             -V/T =  2.0033
             S**2   =   0.0000
 ------------------------------------------------------
 -------------- VARIATIONAL  PCM  RESULTS -------------
<Psi(0)|H|Psi(0)>  (a.u.)= -100.024608
<Psi(0)|H+V(0)/2Psi(0)>  (a.u.)= -100.028947
<Psi(0)|H+V(f)/2Psi(0)>  (a.u.)= -100.029186
<Psi(f)|H|Psi(f)>  (a.u.)= -100.023819
<Psi(f)|H+V(f)/2Psi(f)>  (a.u.)= -100.029187
Total free energy in sol.
(with non electrost. terms (a.u.)= -100.028994
------------------------------------------------------
(Unpol. Solute)- Solvent   (kcal/mol) =     -3.06
(Polar. Solute)- Solvent   (kcal/mol) =     -3.37
Solute polarization        (kcal/mol) =      0.16
Total Electrostatic        (kcal/mol) =     -3.21
------------------------------------------------------
    Cavitation energy      (kcal/mol) =      3.20
    Dispersion energy      (kcal/mol) =     -4.12
    Repulsion energy       (kcal/mol) =      1.04
    Total non electr.      (kcal/mol) =      0.12
 ------------------------------------------------------
    DeltaG (solv)          (kcal/mol) =     -3.09
 ------------------------------------------------------

Note that the PCM results also include the dipole moment in the gas phase and in solution, the various components of the predicted SCRF energy and DG^solvation.

For all iterative SCRF methods, note that the energy to use is the one preceding the Convergence achieved message (i.e.,one from the final iteration of the SCRF method).

INPUT SYNTAX AND KEYWORDS FOR PCM CALCULATIONS
The following keywords are available for controlling PCM calculations (arranged in groups of related items):

SPECIFYING THE SOLVENT
The solvent for the PCM calculation may be specified using the normal Solvent option to the SCRF keyword. The solvent name keyword or ID number may also be placed within the PCM input section. Alternatively, the EPS and RSOLV keywords may be used in the PCM input section to define a solvent explicitly:

EPS=e
Dielectric constant of the solvent.

RSOLV=radius
Solvent radius in Angstroms.

EPSINF=val
Optional value for the dielectric constant at infinite frequency.

CALCULATION METHOD VARIATIONS

NODIS
Skip the calculation of dispersion-repulsion solute solvent interaction.

NOCAV
Skip the calculation of cavitation energy.

DUMP
Provide verbose output as the calculation progresses.

NOSCFVAC
Skip the gas phase calculation before that in solution. While this saves some computation time, it prevents the calculation of DG^solvation, the variation of the dipole moment in solution, and so on.

FIXCAV
Compute the electrostatic energy gradients neglecting the geometrical contributions (i.e., at "fixed cavity"). Be aware that the geometrical contributions are always included in the calculation of non-electrostatic energy gradients (the latter gradients can be skipped with NOCAV and NODIS).

ACCPCM=val
Accuracy threshold for the calculation of electric potential on the cavity. The default value is 10^-6.

TABS=temp
Temperature in Kelvin. The default value is 298.15. Note that you must also specify the correct value of the dielectric constant (episilon) at that temperature using the EPS keyword.

CHARGE NORMALIZATION

ICOMP=N
Specifies the method used to normalize the polarization charge to get the value predicted by Gauss' law. N can take on the following values:

• 1. The difference between the calculated and the theoretical (Gauss) polarization charge is distributed on each tessera proportionally to its area.
• 2. The calculated charge on each tessera is scaled by a constant factor.
• 3. The charge difference is distributed according to the solute electronic density on each tessera.
• 4. The effect of outlying charge is accounted for by means of an additional effective charge, distributed according to the solute electronic density.

Normally, the default is 4 for single point calculations and 2 for geometry optimizations (this one being the only method allowing the calculation of gradients). For CPCM (which is less affected by the outlying charge effects), the default is always 2.

SPECIFYING CHARACTERISTICS OF THE CAVITY

By default, the program builds up the cavity by putting a sphere around each solute heavy atom; hydrogen atoms are always enclosed in the sphere of the atom to which they are bonded; the radius of the atom is increased by a constant amount for each bonded hydrogen atom, up to a maximum of 3 (the increase is0.9 for second-row atoms, 0.13 for third-row atoms, and 0.15 for atoms in higher rows). Explicit hydrogen atoms can be added by specifying the UFF atom type within the molecule specification, as in this example.

RADII=UAHF
Use the United Atom Topological Model [346] to build the cavity, automatically set by the program according to the molecular topology, hybridization, formal charge, etc. Note that hydrogens don't have individual spheres defined and that these radii were optimized for the HF/6-31G(d) level of theory.

RADII=UFF
Use atomic radii from the UFF force field.

RADII=BONDI
Use Bondi¹s atomic radii.

RADII=PAULING
Use Pauling's (actually Merz-Kollman) atomic radii.

ALPHA=scale
Specify the scaling factor (for all the elements but acidic hydrogens) for the definition of solvent accessible surfaces. In other words, the radius of each atomic sphere is determined by multiplying the van der Waals radius by scale. The default value is 1.2.

ALPHAH=hscale
Specify the scaling factor for acidic hydrogens (automatically recognized by the program as those bonded to N, O, P or S halogens). The default value is 1.0.

RET=len
Sets the minimum radius (in Angstroms) of the "added spheres". Increasing this parameter causes the number of added spheres to decrease (for example, to inhibit the creation of added spheres, use RET=100). The default value is 0.2.

NSFE=n
Sets the number of initial spheres (useful if this number is not equal to the number of solute atoms, e. g. if a methyl group is enclosed in one sphere, and so on). When included, the program looks for the positions and the radii of the n spheres in an additional input section following the PCM input (this section is also blank-terminated).

The spheres can be defined using lines in either of the following formats:

N1 [radius [scale hscale]] 
X Y Z [radius [scale hscale]] 	

In the first case, the line specifies an atom number and (optionally) the sphere radius and one or both scaling factors for that atom. In the second case, used to define arbitrary spheres, the Cartesian coordinates of the center of the sphere are again optionally followed by its radius and scaling factors. If parameters are omitted, then the standard values are used automatically. Note that lines of different formats can be freely intermixed in this input section as the program will automatically determine the syntax based on the number and types of parameter specified on any given line.

SELECTING THE POLYDEDRON FOR THE SURFACE TESSERAE

In standard calculations the surface of each sphere is subdivided in 60 triangular tesserae, by projecting the faces of an inscribed pentakisdodecahedron. Other polyhedra with a different number of faces can be used, to get rougher or finer descriptions of the surface, by using one of these keywords:

TSNUM=N
Sets the number of tesserae on each sphere (60 is the default for the usual pentakisdodecahedron). The program automatically selects the polyhedron with the closest number of faces to the specified value of N.

TSARE=area
Specifies the area of the tesserae, in units of Ų (0.4 is a typical value). The program automatically determines the best polyhedron for each sphere.

EXAMPLE PCM INPUT

The following Gaussian job performs a PCM energy calculation on the molecule HF using the solvent cyclohexane. The calculation is performed at a temperature of 300 K using a scaling factor for all atoms except acidic hydrogens of 1.21 and a value of 70 tesserae per sphere:

#
HF/6-31++G(d,p) SCF=Tight SCRF=(PCM,Read,Solvent=Cyclohexane) Test  
PCM SP calculation on hydrogen fluoride  

0,1 
H 
F 1 R  

R=0.9161  

TABS=300.0 
ALPHA=1.21
TSNUM=70   

The final input section ends as usual with a blank line.